In homozygotes, spermatocytes form but fail to undergo meiotic divisions. 64-cell spermatid cysts are absent. BrdU labelling shows that mitotic and premeiotic S phases occur as in wild-type, and meiotic prophase is normal. Subsequent stages of meiosis are aberrant. Chromosomes remain at the nuclear periphery, and centrosome separation occurs but the centrosomes do not reach the poles and do not nucleate asters. The nuclear lamina fails to break down. Spermiogenesis continues to a limited extent. Mitochondria begin to form nebenkern in tetraploid spermatids. Hemizygous males show minor but reproducible defects in spermatogenesis.