Monozygotic twin girls with diploid/triploid chromosome mosaicism and cutaneous pigmentary dysplasia.

Diploid/triploid mosaicism is an uncommon clinical syndrome with a subtle but distinctive phenotype. Characteristic features include prenatal and postnatal asymmetric growth deficiency, triangular and/or asymmetric facies, micrognathia, finger and/or toe syndactyly, clinodactyly, single transverse palmar creases, male genital anomalies, hypotonia and psychomotor retardation. This disorder is underdiagnosed because in 70% of cases the triploid cell line is only seen in fibroblasts. In cases in which a triploid cell line is found in lymphocytes, it usually occurs in less than 5% of cells. While some reports of diploid/triploid mosaicism have mentioned unusual skin pigmentary patterns, including hypomelanosis of Ito, it was only recently recognized that this is a helpful diagnostic clue in mosaic chromosome disorders. We report monozygotic twin girls with diploid/triploid mosaicism whose cutaneous pigmentary dysplasia led to their diagnosis.