GREB1 (growth regulating estrogen receptor binding 1) functions as a crucial mediator of steroid hormone signaling in reproductive tissues and hormone-dependent cancers. The protein acts as a cofactor that amplifies steroid receptor transcriptional output through feed-forward mechanisms with both estrogen and progesterone receptors 1. In endometrial physiology, GREB1 is essential for progesterone-driven stromal cell decidualization, directly interacting with progesterone receptors to regulate target genes like WNT4 and FOXOA1 21. In cancer contexts, GREB1 promotes tumor growth by enhancing androgen receptor DNA binding and recruiting coactivators like EP300 in prostate cancer, contributing to antiandrogen resistance 3. The gene shows differential tissue-specific actions: promoting progesterone responses in normal endometrium while facilitating estrogen-dependent growth in endometriotic lesions 1. GREB1 is considered a potential therapeutic target for hormone-responsive cancers due to its critical role in mediating hormone-stimulated proliferation across multiple cancer types 4. Genetic variants in GREB1 have been associated with female reproductive health conditions, highlighting its importance in reproductive biology 5.