Search PubMed⌕ Search

Biomedical subjects

W L Miller

Publications and source records attributed to W L Miller.

At least 145 records · Page 8Linked to original sources
PubMed1993

Tenascin-X: a novel extracellular matrix protein encoded by the human XB gene overlapping P450c21B.

A human gene termed XB overlaps the P450c21B gene encoding steroid 21-hydroxylase and encodes a protein that closely resembles extracellular matrix proteins. Sequencing of phage and cosmid clones and of cDNA fragments shows that the XB gene spans 65 kb of DNA, consisting of 39 exons that encode a 12-kb mRNA. The predicted protein of over 400 kD consists of five distinct domains: a signal peptide, a hydrophobic domain containing three heptad repeats, a series of 18.5 EGF-like repeats, 29 fibronectin type III repeats, and a carboxy-terminal fibrinogen-like domain. Because the structure of the protein encoded by the XB gene closely resembles tenascin, we term this protein tenascin-X (TN-X), and propose a simplified nomenclature system for the family of tenascins. RNase protection experiments show that the TN-X transcript is expressed ubiquitously in human fetal tissues, with the greatest expression in the fetal testis and in fetal skeletal, cardiac, and smooth muscle. Two adrenal-specific transcripts, P450c21B (steroid 21-hydroxylase) and Y (an untranslated transcript) overlap the XB gene on the complementary strand of DNA, yielding a unique array of overlapping transcripts: a "polygene." In situ hybridization histochemistry experiments show that the TN-X transcript and the P450c21 and Y transcripts encoded on the complementary DNA strand are all expressed in the same cells of the human adrenal cortex. Genetic data suggest that TN-X may be essential for life.

Adrenal Glands↗
PubMed1993

Construction and function of fusion enzymes of the human cytochrome P450scc system.

Type I cytochrome P450 enzyme systems are found in mitochondria and consist of three components, a flavoprotein (adrenodoxin reductase, AdRed), an iron-sulfur protein (adrenodoxin, Adx), and the cytochrome P450; Type II P450 enzymes in the endoplasmic reticulum consist of only two components, P450 reductase and the P450. Genetically engineered fusion proteins of Type II cytochromes P450 (such as steroid 17 alpha- and 21-hydroxylases) produce enzymes with increased activity. To test the consequences of constructing fusions of Type I enzymes, we built fusion proteins based on the cholesterol side-chain cleavage enzyme, P450scc. We constructed expression vectors for three fusion proteins: NH2-P450scc-AdRed-COOH, P450-AdRed-Adx, and P450scc-Adx-AdRed. The various components were assembled from cassette-like cDNA fragments modified and amplified by polymerase chain reaction (PCR), subcloned into a specially tailored vector, and linked by DNA segments encoding hydrophilic linker peptides. The final vectors were transfected into COS-1 cells, incubated with 22R-hydroxycholesterol, and assayed by the secretion of pregnenolone into the culture medium. Triple transfection of three individual vectors expressing P450scc, AdRed, and Adx yielded more pregnenolone than did transfection with P450scc alone. The P450scc-AdRed and P450scc-Adx-AdRed fusion proteins produced levels of pregnenolone similar to the control triple transfection. However, the P450scc-AdRed-Adx fusion produced substantially more pregnenolone, having an apparent Vmax of 9.1 ng of pregnenolone produced per milliliter of medium per 24 hr, compared to a Vmax of 1.7 ng/ml per day for the triple transfection.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenodoxin↗
PubMed1993

Endothelin-mediated cardiorenal hemodynamic and neuroendocrine effects are attenuated by nitroglycerin in vivo.

Endothelin-1 may function pathophysiologically as a counterregulatory vasoconstrictor peptide that is modified in its activity by the opposing action of endothelium-derived relaxing factor(s) (EDRF). The present study determined in part the integrated cardiorenal and endocrine actions of pathophysiologic plasma concentrations of endothelin in the anesthetized dog. In addition, nitroglycerin, which inhibits vascular smooth muscle contraction by increasing cGMP in a mechanism similar to EDRF, acts like an endogenous nitrovasodilator. Therefore, we tested the hypothesis that nitroglycerin would effectively antagonize the cardiac and renal actions of exogenous endothelin. The results confirm that endothelin-1-mediated vasoconstriction in vivo is heterogenous with a greater renal than coronary action. Further, nitroglycerin effectively blocked endothelin-1-mediated coronary flow reductions, but only partially antagonized reductions in renal blood flow. Endothelin-1-induced reduction in cardiac output also was not antagonized by nitroglycerin despite its effects to preserve coronary blood flow. Nitroglycerin did, however, antagonize endothelin-induced elevations in plasma epinephrine, norepinephrine, and aldosterone. These results would suggest that in pathophysiologic states where endothelin-1 is elevated, such as hypertension or congestive heart failure, there is a major compromising of renal function, and also the production of cardiac ischemia. Since exogenous nitroglycerin is relatively ineffective in antagonizing the renal vasoconstrictive effects of endothelin, it may be that the endogenous vasodilating systems, such as ERDF and prostacyclin, are inadequate in such pathologic states to counter the vasoconstrictor effects of endothelin.

Animals↗
PubMed1993

Hospital-based versus community-based clinical education: comparing performances and course evaluations by students in their second-year pediatrics rotation.

BACKGROUND: Increasing use of outpatient settings for clinical education raises the question of their effectiveness compared with that of inpatient settings. METHOD: At the University of Minnesota Medical School-Minneapolis in 1987-88, the 190 second-year students participated in a six-week tutorial rotation introducing them to clinical pediatrics: 52 (27%) were in hospital settings and 138 (73%) were in community outpatient settings. Almost all the students (178) evaluated their rotations by responding to both structured and open-ended questions, using a Likert scale for the structured questions. At the completion of the second year, all 190 students took an objective structured clinical examination (OSCE) that included five pediatrics stations. Student's t-test was used to compare (1) the mean ratings the hospital-based and community-based students gave their pediatrics rotations and (2) the mean scores earned by the two groups of students on the five pediatrics stations in the OSCE. RESULTS: There were no statistically significant differences between (1) the two groups' mean ratings of the clinical experience overall or of the quality of teaching or (2) the groups' mean scores on any of the OSCE stations. CONCLUSION: That the hospital-based and community-based students performed comparably on the OSCE and gave similar evaluations of their pediatrics rotations supports the use of community practitioners to provide students with their initial clinical training. Moreover, community-based teaching sites replicate situations in which most students will eventually practice medicine.

Ambulatory Care↗
PubMed1993

Basal transcriptional activity and cyclic adenosine 3',5'-monophosphate responsiveness of the human cytochrome P450scc promoter transfected into MA-10 Leydig cells.

Mouse Leydig MA-10 tumor cells are a good model of testicular steroidogenesis. The endogenous murine P450scc mRNA in these cells accumulated in response to 8-bromo-cAMP, forskolin, cholera toxin, and 1-methyl-3-isobutylxanthine, but not in response to 1,9-dideoxyforskolin, indicating that this accumulation was stimulated by the protein kinase-A pathway. Inhibiting transcription with actinomycin-D showed that the half-life of cytochrome P450scc mRNA in these cells was not altered by cAMP, consistent with earlier nuclear run-on data showing that the effect of cAMP on P450scc is at the transcriptional level. A series of 17 fragments of 5'-flanking DNA from the human P450scc gene were fused to the gene for firefly luciferase and transiently transfected into MA-10 cells. The longest construct, containing 2327 basepairs of 5'-flanking DNA, responded 4-fold to forskolin and, hence, was used to optimize the forskolin dose response, showing that 30 microM forskolin elicited a 90% maximal effect. Examination of the activity of the deletion constructs located basal and cAMP-responsive sequences. Constructions containing 79 basepairs of 5'-flanking DNA had basal activity; adding sequences between -79 and -110 had minimal effect, but adding sequences between -110 and -127 increased basal activity 3-fold. Adding sequences beyond -127 did not increase basal transcription further, indicating the presence of a basal transcription element between -110 and -127. These serial deletion mutants were used similarly to locate cAMP responsiveness between -1620 and -1676, indicating the presence of a cAMP response element in this region. The locations of these basal and cAMP-responsive sequences correspond well with those previously identified when human P450scc promoter/reporter constructions were transfected into mouse adrenocortical Y-1 cells, but differ from those identified when such constructions were transfected into human JEG-3 choriocarcinoma cells.

1-Methyl-3-isobutylxanthine↗
PubMed1993

Regulation of proteins in the cholesterol side-chain cleavage system in JEG-3 and Y-1 cells.

The conversion of cholesterol to pregnenolone, the rate-limiting step in steroid hormone synthesis, occurs on mitochondrial cytochrome P450scc, which catalyzes this reaction by receiving electrons from NADPH via a flavoprotein [adrenodoxin reductase (AdRed)] and an iron sulfur protein [adrenodoxin (Adx)]. The behavior of the genes and mRNAs encoding these proteins has been studied in several systems, but little is known about the behavior of the human proteins. Using cloned cDNAs for human P450scc and AdRed, we constructed bacterial expression vectors to make milligram quantities of the corresponding proteins. These, plus purified human Adx similarly prepared by Dr. L. Vickery, were injected into rabbits to raise antiserum to each of the proteins. Each antiserum was highly specific and did not cross-react with other mitochondrial proteins detectable by Western blotting. Human JEG-3 choriocarcinoma cells and mouse Y-1 adrenocortical carcinoma cells were then incubated for 0-24 h with 1 mM 8-bromo-cAMP (8Br-cAMP) or 30 nM phorbol 12-myristate 13-acetate (PMA; phorbol ester) plus 1 microM A23187 (calcium ionophore) to activate the protein kinase-A and -C pathways, respectively. In JEG-3 cells, 8Br-cAMP increased and PMA/A23187 slightly decreased the abundance of P450scc and Adx, but neither treatment had a detectable effect on AdRed. The production of pregnenolone by these cells increased 3-fold in response to 8Br-cAMP and fell to one third in response to PMA/A23187. In Y-1 cells, 8Br-cAMP increased the abundance of all three proteins, while PMA/A23187 decreased the abundance of P450scc and Adx. The production of pregnenolone by these cells increased 9-fold in response to 8Br-cAMP and was unaffected by TPA/A23187. These studies show that the three proteins of the cholesterol side-chain cleavage system behave in response to 8Br-cAMP and PMA/A23187 as predicted from the study of their genes and mRNAs, indicating that the chronic regulation of steroidogenesis in these cell systems is regulated principally at the level of mRNA abundance.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1993

Steroid 17 alpha-hydroxylase and 17,20-lyase activities of P450c17: contributions of serine106 and P450 reductase.

Cytochrome P450c17 (EC 1.14.99.9) catalyzes both 17 alpha-hydroxylase and 17,20-lyase activities in mammalian steroidogenesis and also has some 16 alpha-hydroxylase activity. The ratio of 17 alpha-hydroxylase to 17,20-lyase activity differs in the adrenal and testis and is developmentally regulated at adrenarche, but the nature of the enzyme's active site and the differential regulation of its two principal activities are unknown. The spontaneous human P450c17 mutation Ser106-->Pro eliminates all enzymatic activity. We used site-directed mutagenesis to construct expression vectors for the conservative P450c17 mutations Ser106-->Thr and Ser106-->Ala. When expressed in transfected COS-1 cells, these mutants retain only 20-30% of the 17 alpha-hydroxylase and 17,20-lyase activities, but retain 60% of the 16 alpha-hydroxylase activity of the Ser106 wild type. Thus, the amino acid occupying position 106 greatly affects enzymatic activity. Ser is found at position 106 in P450c17 in all mammals and birds studied, but the corresponding residue (position 112) in fish (trout) is Thr. Both the trout Thr112 wild type and a Thr112-->Ser trout mutant had equivalent 16 alpha-hydroxylase, 17 alpha-hydroxylase, and 17,20-lyase activities, although these were only 5%, 5%, and 10%, respectively, of human Ser106. To catalyze its activities, P450c17 must receive electrons from NADPH via a flavoprotein termed P450 reductase. We examined the influence of the ratio of P450c17 to P450 reductase on enzymatic activity by cotransfecting COS-1 cells with varying amounts of vectors expressing each protein. The endogenous P450 reductase of COS-1 cells was sufficient to confer maximal 17 alpha-hydroxylase activity. P450 reductase produced from the transfected expression vector did not increase the conversion of [14C]progesterone to 17 alpha- or 16 alpha-hydroxyprogesterone, indicating that the endogenous immunodetectable P450 reductase of COS-1 cells was sufficient to confer maximal 17 alpha-hydroxylase activity. By contrast, the additional P450 reductase produced by the expression vector increased 17,20-lyase activity about 3-fold. Thus, the availability of reducing equivalents is a crucial factor in regulating 17,20-lyase activity. P450 reductase also increased the 17,20-lyase activity of the Thr106 and Ala106 mutants. These data suggest that the essential role of Ser106 is in the active site, rather than in interacting with P450 reductase, and that electron transfer may play an important role in regulating the 17,20-lyase activity of P450c17.

Aldehyde-Lyases↗
PubMed1993

Structure and function of the hepatic form of 11 beta-hydroxysteroid dehydrogenase in the squirrel monkey, an animal model of glucocorticoid resistance.

Both cortisol and aldosterone bind to and activate the mineralocorticoid receptor. Cortisol concentrations are generally 100- to 200-fold higher than aldosterone concentrations, yet mineralocorticoids clearly exert effects different from glucocorticoids. One hypothesis is that 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD), which converts cortisol to biologically inactive cortisone, protects the mineralocorticoid receptor from cortisol. The circulating concentrations of cortisol in the squirrel monkey are 20- to 50-fold higher than human cortisol concentrations, yet this animal has no evidence of glucocorticoid or mineralocorticoid excess. We used this experiment of nature to test the hypotheses that the known (hepatic) form of 11 beta-HSD protects renal mineralocorticoid receptors from the action of cortisol and that it modulates glucocorticoid concentrations in target tissues. Using a long oligonucleotide based on the rat sequence, we cloned the squirrel monkey 11 beta-HSD complementary DNA and gene. The encoded monkey amino acid sequence is 75% and 91% identical to the corresponding rat and human sequences, respectively. The tissue abundance of the messenger RNA for the monkey enzyme was similar to or less than that seen for the rat and human enzymes. Both the monkey and human 11 beta-HSD complementary DNAs were cloned into an expression vector and used to transfect cultures of Chinese hamster ovary cells. Both vectors were transcribed and translated into equivalent amounts of 11 beta-HSD enzyme. The monkey enzyme was slightly more efficient than the human enzyme in converting [3H]cortisol to cortisone, and estimates of the Michaelis-Menten constant and maximum velocity of both enzymes are similar. These data indicate that the abundance and activity of the hepatic form of 11 beta-HSD are insufficient to inactivate the very high concentrations of cortisol in the squirrel monkey, suggesting that this form of 11 beta-HSD does not defend the mineralocorticoid receptor or protect tissues from high cortisol concentrations. Rather, this enzyme appears to favor conversion of cortisone to cortisol, thus maximizing tissue concentrations of cortisol to overcome glucocorticoid resistance associated with a 50% reduction in glucococorticoid receptors.

11-beta-Hydroxysteroid Dehydrogenases↗
PubMed1993

Deletion of amino acids Asp487-Ser488-Phe489 in human cytochrome P450c17 causes severe 17 alpha-hydroxylase deficiency.

17 alpha-Hydroxylase deficiency blocks the biosynthesis of cortisol and sex steroids, resulting in mineralocorticoid excess, hypertension, sexual infantilism, and female phenotype in both genetic sexes. The disease is caused by mutations in the gene encoding cytochrome P450c17, which is the single enzyme that mediates both 17 alpha-hydroxylase and 17,20-lyase activities. We report a 14-yr-old patient from Thailand with a classical clinical presentation of this rare disorder. Analysis of her P450c17 gene by polymerase chain reaction amplification and sequencing showed a nine-base deletion, eliminating codons 487-489 (Asp-Ser-Phe) near the carboxy-terminus of P450c17. This deletion creates a BclI site in the mutant DNA, permitting accurate demonstration that the patient was homozygous for this lesion, whereas one parent and two siblings were heterozygous. By use of site-directed mutagenesis, we created a vector that could express this mutated form of P450c17 when transfected into non-steroidogenic COS-1 cells. Such transfected cells produced immunodetectable P450c17 protein, but had no 17 alpha-hydroxylase or 17,20-lyase activity, whereas cells similarly transfected with a vector expressing normal human P450c17 could 17 alpha-hydroxylate either pregnenolone or progesterone and convert 17 alpha-hydroxypregnenolone to dehydroepiandrosterone, showing the presence of both activities. This is the first report of the molecular genetic basis of 17 alpha-hydroxylase deficiency in a Southeast Asian patient.

17-Hydroxycorticosteroids↗
PubMed1993

Regulation of steroidogenesis in NCI-H295 cells: a cellular model of the human fetal adrenal.

NCI-H295 is a recently described human adrenocortical carcinoma cell line that makes a variety of steroid hormones. We sought to determine if steroidogenesis in these cells employs the same enzymes as those used in normal adrenal steroidogenesis, and if the genes encoding those enzymes exhibit characteristic responsiveness to activators of the protein kinase-A and -C pathways of intracellular second messengers. Northern blots show that NCI-H295 cells contain abundant mRNAs for three key steroidogenic enzymes, cytochrome P450scc, cytochrome P450c17, and cytochrome P450c21. These mRNAs accumulated in a time- and dose-dependent fashion in response to 8-bromo-cAMP (8Br-cAMP), forskolin, cholera toxin, and 3-isobutyl-1-methylxanthine, all activators of the protein kinase-A pathway. Nuclear run-on assays and actinomycin-D transcriptional inhibition experiments show that cAMP regulates the expression of all three genes primarily at the transcriptional level. Inhibition of protein synthesis with cycloheximide did not prevent the cAMP-induced accumulation of P450scc or P450c17 mRNAs, but did inhibit accumulation of P450c21 mRNA, suggesting that cAMP is acting through a mechanism dependent on protein synthesis to promote accumulation of P450c21 mRNA. Stimulation of the protein kinase-C pathway with phorbol ester decreased P450scc and P450c17 mRNAs, but stimulated the accumulation of P450c21 mRNA. RNase protection experiments, Northern blot hybridizations, and reverse transcription-polymerase chain reaction show that NCI-H295 cells express both the 11 beta-hydroxylase (P450c11 beta) encoded by the P450c11B1 gene and the aldosterone synthetase (P450c11AS) encoded by the P450c11B2 gene. 8Br-cAMP increased the abundance of both of these mRNAs with similar kinetics, with maximal accumulation of both after about 24 h. NCI-H295 cells also contain the mRNAs for aromatase and insulin-like growth factor-II. 8Br-cAMP increased the abundance of aromatase mRNA and decreased the abundance of IGF-II mRNA. These studies show that NCI-H295 cells express most of the enzymes needed for human adrenal steroidogenesis, and that the genes encoding these enzymes respond to stimulation of second messenger pathways in a manner similar to that of human adrenals. NCI-H295 cells appear to be a good model for studying the molecular regulation of human adrenal steroidogenesis.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1993

Transcriptional regulation of human genes for steroidogenic enzymes.

Steroid hormones are synthesized from cholesterol in the adrenals, gonads, and placenta by a complex series of reactions. The human genes encoding each of these biosynthetic enzymes have been cloned, permitting study of their regulation. Tropic hormones, such as corticotropin and the gonadotropins, exert their chronic effects on steroidogenesis by increasing the amounts of steroidogenic enzymes; this in turn occurs primarily through increased gene transcription. Our studies have emphasized the cholesterol side-chain cleavage enzyme P450scc, which catalyzes the first and rate-limiting step in steroidogenesis, and P450c17, which determines what class of steroids is synthesized. By fusing the promoters of the genes for these enzymes to readily assayed reporter genes and transiently transfecting cultured cells with these constructions, we have identified the regions of each promoter that confer basal expression, induction by cAMP, and repression by activators of protein kinase C. Different segments of the P450scc promoter are used for each of these purposes in different cell types, indicating that the regulation of this gene is very complex. Transcription is not the only level at which steroidogenesis is regulated. The abundance of mRNA for adrenodoxin reductase, a flavoprotein needed for P450scc activity, is post-transcriptionally regulated by cAMP.

Base Sequence↗
PubMed1992

cAMP post-transcriptionally diminishes the abundance of adrenodoxin reductase mRNA.

Adrenodoxin reductase (AR; ferridoxin: NADP+ oxidoreductase, EC 1.18.1.2) is a flavoprotein that mediates electron transport from NADPH to all known mitochondrial forms of cytochrome P450. AR mRNA was found in all human adult and fetal tissues examined; however, it was vastly more abundant in tissues that synthesize steroid hormones. The ratio of the 18- form of mRNA lacking 18 alternately spliced bases to the 18+ form was approximately 100:1 and remained constant irrespective of the tissue or hormonal manipulation, indicating that the alternate splicing is a passive nonregulated event. AR protein was unchanged by forskolin treatment of human JEG-3 cytotrophoblast cells for 24 h, but the mRNA diminished. Phorbol 12-myristate 13-acetate and cycloheximide had no effect, even though these agents had the expected effects on P450scc and adrenodoxin mRNAs. cAMP decreased the abundance of AR mRNA expressed from both transfected plasmids and the endogenous gene, indicating the effect was post-transcriptional. AR gene transcription in JEG-3 cells and promoter-chloramphenicol acetyltransferase constructs transfected into JEG-3 cells were unresponsive to forskolin. Powerful basal transcription elements were identified between -46 and -214 bases from the principal transcriptional initiation site, a region containing six elements closely resembling the binding site for transcription factor SP1.

Cells, Cultured↗
PubMed1992

Rainbow trout cytochrome P-450c17 (17 alpha-hydroxylase/17,20-lyase). cDNA cloning, enzymatic properties and temporal pattern of ovarian P-450c17 mRNA expression during oogenesis.

A cDNA clone encoding cytochrome P-450c17 (17 alpha-hydroxylase/17,20-lyase) was isolated from a rainbow trout ovarian follicle cDNA library. The cDNA contained an open reading frame of 1,542 nucleotides encoding a protein of 514 amino acid residues. The amino acid sequence of trout P-450c17 shows a much greater homology with chicken P-450c17 than with that of human, bovine and rat. The trout P-450c17 expressed in non-steroidogenic mammalian COS-1 cells showed both 17 alpha-hydroxylase and 17,20-lyase activities. The cDNA only hybridized to a single species of mRNA (2.4 kb) isolated from rainbow trout ovaries; the 2.4 kb transcripts were abundant in trout ovaries during the later stages of oogenesis.

Aldehyde-Lyases↗
PubMed1992

Infection of cultured human adrenal cells by different strains of HIV.

OBJECTIVE: To determine whether human adrenal cells can be infected by HIV. METHODS: Cultured human fetal adrenal cells and the SW13 human adrenocortical carcinoma cell line were inoculated with several HIV-1 and HIV-2 strains. Virus replication was detected by viral core antigen enzyme-linked immunosorbent and reverse transcriptase assays. CD4 expression was measured by Northern blot and polymerase chain reaction procedures. RESULTS: HIV infection of these adrenal cells was detected and was most evident after cocultivation of the inoculated cells with peripheral blood mononuclear cells. Infection does not involve the CD4 molecule, which is not expressed by these adrenal cells. The relative level of HIV replication depended on the viral strain used. Virus production occurred best in cells that maintained evidence of adrenal cell function. Infection did not appear to disturb steroidogenesis measured in the cells. CONCLUSIONS: These observations indicate that human adrenal cells are susceptible to HIV infection, and provide further evidence of the polytropic nature of the virus.

Adrenal Cortex↗
PubMed1992

Mechanism and consequences of the duplication of the human C4/P450c21/gene X locus.

The adjacent C4 and P450c21 genes encode the fourth component of serum complement and steroid 21-hydroxylase respectively, and are tandemly duplicated in the human, murine, and bovine genomes. We recently cloned a cDNA for another duplicated gene, operationally termed X, which overlaps the 3' end of human P450c21 and has the opposite transcriptional orientation. Thus, the organization of the locus is 5'-C4A-21A-XA-C4B-21B-XB-3' (Y. Morel, J. Bristow, S. E. Gitelman, and W. L. Miller, Proc. Natl. Acad. Sci. USA 86:6582-6586, 1989). To determine how this locus was duplicated, we sequenced the DNA at the duplication boundaries and the 7 kb between P450c21A and C4B comprising the XA locus. The sequences located the duplication boundaries precisely and indicate that the duplication occurred by nonhomologous recombination. The boundaries are substantially different from those of the corresponding duplication in the mouse genome, suggesting that similar gene duplications may have occurred independently in ancestors of rodents and primates after mammalian speciation. Compared with XB, the XA gene is truncated at its 5' end and bears a 121-bp intragenic deletion causing a frameshift and premature translational stop signal. Nevertheless, XA is transcribed into a stable 2.6-kb polyadenylated RNA that is expressed uniquely in the adrenal gland.

Adrenal Glands↗