CHEK1 and Infertility: However, since Chek1 CKO spermatocytes complete prophase I and differentiate into spermatozoa, with only minor defects such as a delay in the removal of γH2AX from autosomes (Abe et al., 2018), the observed defect in CHK1 S317 phosphorylation is unlikely to be the cause of the infertility observed in Topbp1 B5 mutants.