In 2011, Szczepańska et al. concluded that single nucleotide polymorphisms in genes encoding folate metabolism enzymes may contribute to endometriosis-associated infertility through an epistatic interaction of the rs1801133 of MTHFR (C677T) and rs4244593 of phosphatidylethanolamine N-methyltransferase (PEMT) [18]. Here, MTHFR is linked to endometriosis.