The lines of Blaschko represent a nonrandom developmental pattern of the skin fundamentally differing from the system of dermatomes. Many nevoid skin lesions display an arrangement following these lines. This is a review of case reports providing photographically documented evidence that the lines of Blaschko become manifest in the heterozygous state of various X-linked gene defects such as incontinentia pigmenti, focal dermal hypoplasia, X-linked dominant chondrodysplasia punctata, X-linked hypohidrotic ectodermal dysplasia, and Menkes syndrome. Hence, a causal relationship between lyonization and the lines of Blaschko seems quite obvious. Although it should be borne in mind that other genetic mechanisms such as somatic mutations or chimerism may give rise to the same linear pattern, the datable embryologic event of X-inactivation seems most suitable to explain the origin and nature of the lines of Blaschko. Apparently, in women affected with X-linked skin disorders the lines of Blaschko visualize the clonal proliferation of two functionally different populations of cells during early embryogenesis of the skin. The typical dorsal V-shape and the abdominal S-figure of these lines may result from an interference of the transversal coherent proliferation with the longitudinal growth and flexion of the embryo. In contrast to Blaschko's original assumption, it is now clear that these lines are independent from the metameric structure of the human body. Obviously, they represent a marker of the normal development of human skin. Therefore, a thorough study of the distribution pattern of X-linked skin disorders in women may give us a better insight into the early embryogeny of the human integument.