Abstract: We report a 46,XY boy with a mild focal dermal hypoplasia phenotype who had both wild-type and mutated copies of the PORCN gene and was, therefore, mosaic for the mutation. He had cutaneous syndactyly, hydronephrosis, and nail dystrophy. Small whitish depigmented spots, which were slightly depressed from the skin surface, were distributed linearly on the trunk and arms. Aside from these findings, streaks of brown-pigmented macules were seen on the dorsal aspect of the legs. Both the linear arrangement of the whitish spots and the streaks of pigmented macules followed the lines of Blaschko. The phenotype of the patient, who did not exhibit cribriform atrophy, telangiectasia or fat herniation, seemed to be much milder than that of typical female patients with focal dermal hypoplasia. Analysis of the genomic DNA extracted from the peripheral lymphocytes revealed a transition 129G>A within exon 1 of PORCN, which leads to a nonsense mutation W43X. The percentage of peripheral lymphocytes carrying a mutation was estimated to be 50% by the subcloning and sequencing of individual clones of the PCR product amplified across the mutation. This patient’s case history provides further molecular evidence supporting the concept that “male focal dermal hypoplasia” does exist and that typical features such as telangiectasia and fat herniation are sometimes absent.