ESR1 and polycystic ovary syndrome: After PCOS patients enter the secretory phase, the expression of the ER activator AIBI and the transcription mediator TIF2 in the endometrium is markedly increased, further activating ERα, which upregulates E in patients and enhances the long-term stimulation effect of E. The vicious cycle of the two affects the establishment of benign endometrial receptivity [67].