Extracutaneous analogies of Blaschko lines

In females, early random X-chromosome inactivation in the late blastocyst and subsequent embryonic development cause a random distribution of cells with an active paternal or maternal X-chromosome. Carriers of X-linked disorders are mostly healthy but, when thoroughly examined, may display a characteristic pattern of partial involvement, which for the skin, is known to follow the lines of Blaschko. Comparable patterns of involvement have been seen in various other organs. The evaluation of carriers who are heterozygous for X-linked disorders, therefore, is an efficient method for the study of functional genetic mosaicism in humans. The same patterns can also be seen in case of early embryonic somatic mutations.