ESR1 and endometriosis: In contrast to ERα, the low levels of the ESR2 mRNA and ERβ protein in the eutopic endometrium of women with moderate to severe OE compared with healthy women observed in the present study are consistent with the low ERβ levels reported in cells of the eutopic endometrium from patients with endometriosis, which were positively correlated with increased telomerase expression that indicated a persistently greater proliferative phenotype [71, 72].