Gatekeeper of meiotic initiation in both male and female germ cells (PubMed:32032549). In complex with STRA8, directly activates the transcription of a subset of critical meiotic genes playing a central role in cell-cycle switching from mitosis to meiosis. Temporal expression of MEIOSIN is required for meiotic entry decision (PubMed:32032549). Interacts with STRA8. Specifically expressed in adult testis and embryonic ovary. Induced in 13.5 dpc in the embryonic ovary, expression persists at least until 18.5 dpc (PubMed:32032549). In testes, expressed in spermatocytes during the preleptotene stage in the stage VII-VIII seminiferous tubules (at protein level) (PubMed:32032549). Mutant mice develop normally. Males show defects in reproductive organs with smaller-than-normal testes (PubMed:32032549). They have severely impaired spermatogenesis and the absence of postmeiotic spermatids or sperm in testes and epididymis (PubMed:32032549). Female ovaries are degenerated with apparently fewer mature follicles at the age of 4 weeks old (PubMed:32032549).