[Phacomatosis pigmentovascularis interpreted as a phenomenon of twin spots].

A case of phacomatosis pigmentovascularis is reported. In order to explain the origin of this phenotype, we propose the genetic concept of twin spots. The teleangiectatic birthmark and the pigmentary naevus are caused by two different recessive mutations. Their loci are situated on the same chromosome. The embryo is doubly heterozygous, bearing one of the two mutations on one chromosome, whereas the other mutation is present on the homologous chromosome. During embryogenesis somatic crossing-over occurs, resulting in two different cell populations, each being homozygous for one of the two mutations. The mechanism of mitotic recombination would be a decisive event establishing an aetiological link between the two different naevi, thus making them twin spots.