We describe three patients with the cutaneous manifestations of hypomelanosis of Ito. Two, with unusual abnormalities of their toes, had a mixture of diploid and triploid cells in cultured skin fibroblasts. The published clinical descriptions of hypomelanosis of Ito and diploid-triploid mosaicism are reviewed. Chromosome heteromorphisms, HLA types, and DNA fingerprints were studied in an attempt to elucidate the origin of the disease in our patients. We conclude that hypomelanosis of Ito is a manifestation of a heterogeneous group of disorders, the common factor being the presence of two genetically different cell lines. It can result from chromosomal mosaicism or chimerism, from a postzygotic mutation, or from X inactivation. The risk of recurrence is negligible if the proband is a male; if the proband is female the risk is also low but an X linked mutation must be considered.
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