Ring chromosome 21: characterization of DNA sequences at sites of breakage and reunion.

We have presented studies of an unusual child with an r21 chromosome who lacks the phenotype of Down syndrome. We have sequenced the region of the breakpoint in the normal DNA fragment and have isolated the abnormal breakpoint fragment as a 7.5-kb EcoRI fragment. We have preliminary evidence localizing the breakpoint to a few hundred base pairs of 21q DNA. Since the child lacks the classical phenotype of Down syndrome, further studies of the DNA distal to the breakpoint on the long arm of chromosome 21 may help us to elucidate "genes" important to the phenotype of Down syndrome.