ESR1 and ovarian cancer: Compared with patients who exhibited progression of ovarian cancer, tumors from stable patients (using CA125 marker criteria of which response is associated with ER expression in a phase II trial of letrozole in ovarian cancer) showed significantly lower expression levels of several estrogen-responsive genes, which were demonstrated to be ERα-dependent and upregulated by E2 in the PEO4 ovarian cancer cell line; moreover, higher aromatase expression has been observed in stable patients (150, 151).