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Although pediatric solid tumors are cytogenetically less well characterized than childhood leukemias, an understanding of the role of chromosomal changes in the development of these neoplasms is emerging. The major clinical importance of chromosome analysis today is diagnostic. Especially in small cell round cell tumors of childhood, the unique karyotypic patterns that characterize some of the differential diagnostic entities make it possible to determine with a high degree of certainty which type of cancer the child has. Molecular studies have revealed that almost all retinoblastomas show homozygous loss of function of the RB1 gene in 13q14. At the cytogenetic level, however, aberrations of 13q are seen in less than 25% of retinoblastomas; instead, the presumably progression-related i(6p) and aberrations leading to gain of 1q predominate, each being present in one-third of the tumors. Twenty percent of cytogenetically aberrant Wilms' tumors show structural rearrangements, often deletions, of 11p13 and 11p15, where the WT1 and WT2 genes map. Other frequent changes are trisomy 12 and duplication of 1q. The most common (80%) cytogenetic abnormality in neuroblastoma is loss of distal 1p, a chromosome segment thought to harbor at least two tumor-suppressor genes of importance in tumorigenesis. Double minute chromosomes or homogeneously staining regions are present in one-third of all neuroblastomas and are associated with MYCN amplification. Loss of 1p material or MYCN amplification predicts a poor outcome. The most common (30%) chromosomal aberration in primitive neuroectodermal tumors of the central nervous system is i(17q). The formation of this isochromosome may help inactivate a tumor-suppressor gene located distal to the TP53 locus on 17p. No specific chromosome abnormality has been detected in gliomas, but monosomy 22 and rearrangements leading to loss of 1p and gain of 1q are recurrent. Few hepatoblastomas with chromosomal changes have been reported, but several potential primary aberrations have been described, including +2, +20, and duplication 8q. In Ewing's sarcoma, t(11;22)(q24;q12) is the primary aberration, with trisomy 8 and gain of 1q being frequent secondary changes. Fibrosarcomas in children often carry only numeric aberrations, especially trisomy for chromosomes 11, 20, 17, and 8. Most osteosarcomas are cytogenetically complex, and no specific abnormality has been detected; the single most common change is loss of chromosome 13, which is observed in half the tumors. In contrast, the low-malignancy parosteal osteosarcomas often display supernumerary ring chromosomes as the sole karyotypic deviation. The cytogenetic profiles of rhabdomyosarcomas differ among the various morphologic subtypes.(ABSTRACT TRUNCATED AT 400 WORDS)