The predominant cause of spermatogenic arrest of meiosis is the failure of homologous chromosomes to accurately synapse. MEI1 (Meiosis inhibitor protein 1), also designated Meiosis defective protein 1, is a 1274 amino acid protein that is likely required for the formation of genetically programmed double-strand breaks, the first step in the initiation of meiosis. With predominant expression in testis, it is likely that defects of the gene encoding MEI1 results in male infertility. Interestingly, studies show that genetic variation in the MEI gene possibly predisposes European Americans but not Israeli men to infertility by meiotic arrest. Human MEI1 shares 79% sequence similarity with its mouse homolog. There are seven isoforms of MEI1 that are produced as a result of alternative splicing events.