De novo increase of E2 in endometriosis lesions affects the ratio of Erα and Erβ, thus impacting inflammation and the expression of some target genes, such as Growth regulation by estrogen in breast cancer 1 (Greb-1) protein and multifunctional transcription factor oncogene (c-Myc), that result in endometriosis’ progression [20,21]. The gene discussed is GREB1; the disease is endometriosis.