21 ring chromosome in a girl with stigmata of Down's and G deletion I syndromes.

An 11-year-old girl with severe psychomotor retardation and other congenital anomalies characteristic of Down's and G deletion I syndromes, was confirmed to be a chromosome 21 ring carrier by R- and G-banding techniques. The dynamics of the ring evolution resulting in a population of complex unstable rings, as in our index case, and the effects of the consequent ring mosaicism are briefly discussed.