Previous research has recognized the vital role played by the PGR-induced Indian hedgehog—chicken ovalbumin upstream promoter-transcription factor II—WNT family member 4 (IHH-COUPTFII-WNT4) pathway in regulating epithelial proliferation and decidualization during early pregnancy, while the expression levels of all three proteins decrease in the endometrium of endometriosis patients, leading to endometrial non-receptivity [32,33,34]. The gene discussed is WNT4; the disease is endometriosis.