ESR1 and endometrial carcinoma: Given that PGR and KLF9 expression in term myometrium is essential for labor onset, and the ability of KLF9 to act as a negative regulator of ligand-dependent ESR1 signaling (e.g., Ishikawa endometrial carcinoma cells) (49), KLF9 and related family members KLF4 and KLF13 may serve to link PGR- and ESR1-mediated RLN/RXP1 actions in the peri-parturient myometrium.