[X-linked genodermatoses].

Chromosome X is one of the best genetically defined. Many disease loci are assigned to this chromosome, due to the peculiar mode of inheritance of X-linked disorders. Chromosome X undergoes X-inactivation in females. Recombination with chromosome Y occurs at pseudoautosomal regions. Some features of X-linked genodermatoses are a consequence of these phenomenons: variable expression, topography following Blaschko's lines. This can be seen in incontinentia pigmenti, focal dermal hypoplasia or hypohidrotic ectodermal dysplasia. Deletions at the pseudoautosomal region may cause contiguous gene syndromes. Hence ichthyosis with steroid-sulfatase deficiency may occur in association with various disorders. Transmitting females should be recognized by clinical examination or molecular studies, as this represents the main point in genetic counselling.