Trisomy 18 mosaicism: clues to the diagnosis.

Karyotypes of blood and skin fibroblasts at ages 3 and 8.5 years had shown non-mosaic trisomy 18 in a male now of age 19. Because of his prolonged survival and an atypical phenotype, skin fibroblast cultures from a new biopsy were established at age 18, and only normal 46,XY cells were observed, while peripheral blood lymphocytes still demonstrated 47,XY, + 18. This patient and six others with trisomy 18 mosaicism illustrate the advisability of looking for such a pattern in individuals whose phenotype in early life is not fully consistent with the trisomy 18 syndrome. Additional clues to the presence of trisomy 18 mosaicism are male sex, survival beyond 2 years and lack of fingertip arches.