Required for normal meiotic chromosome synapsis. May be involved in the formation of meiotic double-strand breaks (DSBs) in spermatocytes. Strongly expressed in testis, weakly in brain, and not detected in spleen, liver, kidney, small intestine or colon. Highly expressed in 17 dpc ovaries. Expression is significantly reduced in newborn ovaries, and is undetectable in ovaries from 5 day and 8 week old mice. Mice are infertile due to meiotic arrest caused by defects in chromosome synapsis. They lack zygotene Rad51 foci, and are likely to be deficient in meiotic double-strand break (DSB) formation. In addition, mutant spermatocytes exhibit meiotic arrest at entry into pachynema, whereas oocytes progress to an abnormal metaphase I. MEI1-deficient oocytes are capable of fertilization. The zygotes are capable of initiating embryonic development but mostly arrest at the 2-4 cell stage. Among all the zygotes, approximately 5% are androgenetic and do not contain maternal chromosomes (PubMed:30388401).