ESR1 and polycystic ovary syndrome: Here, we report a PCOS patient harboring a novel missense ESR1 mutation, who had normal basal serum estradiol and LH levels and elevated FSH and AMH, levels and underwent a markedly prolonged ovarian stimulation period and poor response to this stimulus after pituitary downregulation with GnRH-a, exhibiting characteristics of estrogen insensitivity during the COH phase of an IVF cycle.