Allelic somatic mutations may explain vascular twin nevi.
Vascular twin nevi, i.e., telangiectatic nevus and nevus anemicus occurring together and adjacent to each other, can be explained as twin spots resulting from a somatic recombination. It is so far unclear, however, whether the postulated underlying autosomal recessive mutations are allelic. This problem can be approached by studying another cutaneous phenotype, phacomatosis pigmentovascularis. Within this diagnosis, several authors have reported the simultaneous occurrence of three different birthmarks, viz., a pigmentary nevus, a telangiectatic nevus and a nevus anemicus. These cases can also be explained as a twin-spot phenomenon, provided two of these nevi are considered allelic traits. The two vascular nevi are most probably allelic, whereas the pigmentary nevus may be a nonallelic component of this syndrome. In conclusion, phacomatosis pigmentovascularis provides circumstantial evidence suggesting allelism of somatic mutations giving rise to two different vascular nevi.