A diagnosis of IFAP (ichthyosis follicularis
with atrichia and photophobia) syndrome was established in a 1-year-old boy with congenital hairlessness, generalized ichthyotic skin changes with follicular hyperkeratoses, and photophobia. IFAP syndrome is considered to be an X-linked recessive trait. The phenotype present in female carriers has so far not been delineated. A 2-year-old sister had atrophoderma and ichthyotic skin lesions arranged in a linear pattern and a large noncicatrical bald patch on her scalp. Similarly, the mother had linear lesions of scaling and atrophy as well as circumscribed hairless areas involving the scalp, the axillary region, and the lower legs. Sweat testing by means of iodine starch-reaction visualized hypohidrotic linear lesions corresponding to the areas of hyperkeratosis and atrophy. In both mother and daughter the lesions followed the lines of Blaschko, whereas the boy was diffusely affected. Family history showed that the boy's maternal uncle who had died at age 1 year was likewise affected with the same disorder. Moreover, the maternal grandmother had reportedly bald patches on her scalp and very dry skin. This is the first report to document linear skin lesions visualizing lyonization in women heterozygous for