ESR1 and endometriosis: To date, the only two published studies investigating the role of estrogen metabolites in endometriosis concluded that 2-hydroxyestradiol and 2-methoxyestradiol reduced endometriotic cell growth via estrogen receptor independent mechanisms [39]; and endometriosis metabolizes estrogen preferentially to the biologically active of 2-hydroxyestradiol, and genotoxic 4-hydroxyestradiol and 4-hydroxyestrone metabolites [40].