Two cases of ring chromosome 13. Chromosome banding patterns and mosaic configuration.

Two ring chromosomes 13 were studied by means of various cytogenetic techniques in two psychomotorically retarded male infants. Common features of our patients include microcephalia, hypertelorism, wide and prominent nose bridge, and cryptorchism. Various configurations of the aberrant chromosomes could be identified in cultured skin fibroblasts and peripheral lymphocytes from both patients. Chromosome heteromorphisms and analysis of silver stained nucleolus organizer regions (Ag-NOR) substantiated the parental origin of the ring chromosomes. The more severely affected patient showed a break point at band 13q32 in the long arm of the ring, whereas in the less severely affected the loss of material during ring formation was restricted to the telomere. This provides further evidence for a clinical relevance of the detected mosaic configurations of the rings.