We report a case of Down syndrome with unusual chromosome translocatio
n. The proband is a 1 year old male with 47, XY, der (5) t(5;21) (q11;
q21), + der (5) t(5;21). The mother's karyotype was 46, XX, der (5) t(
5,21) (q11;q21). The chromosome imbalance resulted from 3:1 meiotic se
gregation.