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Abstract: A 9-year-old boy complained of increased freckling on the extremities since very early infancy. Examination showed mottled pigmentation with areas of depigmentation on the backs of the hands and tops of the feet, and less strikingly on the arms and legs. Scattered, small, pigmented freckles on the face were also noticed. The condition was clinically diagnosed as dyschromatosis symmetrica hereditaria (DSH) at first. Unscheduled DNA synthesis was reduced to 66% of the normal value, however, and an ultraviolet sensitivity test by colony formation on fibroblasts revealed moderate sensitivity. Therefore this case was finally diagnosed as xeroderma pigmentosum. It is necessary to study cellular repair capacity to make a precise diagnosis when the distinction is so difficult.