MTHFR and Infertility: The male knockout mouse of an enzyme in the pathway, methylene tetrahydrofolate reductase (MTHFR), which converts 5,10-methylene tetrahydrofolate (THF) to 5-methyl THF and promotes the conversion of homocysteine to methionine, showed high homocysteine levels, reduced germ cell counts, impaired spermatogenesis, and infertility [Frosst et al., 1995].