A family with three sibs carrying trisomy 21.

A family with three sibs, including a pair of dizygotic twins, all affected by Down's syndrome with regular trisomy 21, is described. The chromosome counts carried out on prolonged fibroblasts cultures of the mother, revealed the presence of the trisomy 21 in 6 out of 688 scored mitoses. The cytological findings give support to the hypothesis of a chromosome mosaicism in one of the normal parents, as a cause of the recurrence of the trisomy 21.