A study of the ring chromosome 13 syndrome is presented with detailed clinical and cytogenetic features of three new unrelated cases. The clinical limits of this syndrome can now be defined. An analysis of these cases together with those in the literature indicates that the syndrome forms a continuous spectrum, and no further taxonomic subdivision is possible at this stage of knowledge. The chromosome breakpoints in the first two cases are 13p11 and 13q32 and in the third case 13p11 and 13q33 or 13q34. All described cases of the ring 13 syndrome have breakpoints within the region bounded by bands 13q21 to 13q34. All rings are negative for silver banding. Peripheral blood cultures showed an average of 88 of metaphases to be 46.XX,r(13), with the remaining 12 manifesting either random loss or ring duplication. The rings vary in size and show a variable number of centromeres. An estimate of the birth incidence of this condition in the Anglo-Saxon population is 1 in 58,000. Parents of affected children are clincally and cytogenetically normal, the rings in affected offspring being meiotic in origin.
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