The ectopic endometrium is biologically the same as basal intrauterine endometrial tissue and – because endometriosis cells express estrogen receptors (ERα, Erβ, and GPER) and P4 receptors (PR-A and PR-B) – grows and undergoes cyclic proliferation and breakdown like the ectopic endometrium (28, 29). This evidence concerns the gene ESR2 and endometriosis.