A 46, XYq+, r(B) karyotype was observed in a four-month-old boy with cri du chat syndrome. The ring was identified as a No. 5 on the basis of both G-staining and quinacrine fluorescence. The break points were located at bands 5ρl5 and 5q35, i.e., his karyotype was 46, XYq+ , r(5)(pl5q35). The loss of chromosome material was estimated to be about 5.5% of a normal No. 5 chromosome. The father also had a Yq+ chromosome