ESR1 and breast cancer: Based on our findings in breast cancer patients in the METAMORPH study that PAQR8 undergoes preferential CN gain in recurrent tumors, and that PAQR8 CN gain is mutually exclusive with activating mutations in ESR1 (Fig. 1A) [3], we sought to determine whether PAQR8 gain was associated with clinical progression in this patient cohort [3].