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An average-sized human Y chromosome contains about 3,000 copies of the repeating DNA family DYZ1. A major repeating unit of the family, pHYlO, has been cloned and an entire 3,564-bp sequence has already been determined by Nakahori et al. (1986). In the present study, pHYlO was divided into six consecutive segments, A to F, which were independently amplified by the PCR technique to see if they were strictly male-specific. pHYlO appears to consist of segments of various male-specificity. The B segment was apparently male-specific; however, the use of additional techniques (Southern-blot analysis or second PCR amplification in combination with the standard PCR) revealed homologous sequences in some females. None of the six segments of pHYlO may be male-specific in a strict sense. Different segments appear to be conserved during evolution to different extents. The 323-bp E segment appears to be the least conserved and to be responsible for the generation of most variations within the DYZ1 family.

Treatment of standard blood cultures with acridine orange (20–25 ug/ml) during the final 30 min of culture, 30–90 min after the addition of Colcemid, provides a simple method for obtaining elongated human chromosomes for high-resolution trypsin-Giemsa banding.

G-group chromosomes involved in satellite associations were identified by the technique of G-banding. Metaphases were obtained from nine normal individuals. Out of a total of 537 G-group chromosomes, 297 were No. 21s and 240 were No. 22s. There seemed to be a slight but consistent excess of No. 21 chromosomes in satellite associations.

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.