ESR1 and breast cancer: For 63 patients (41 and 22 in training and test sets, respectively) with available receptor status for matched primary breast cancer and BCBM, an overall conversion rate of 57% (36/63) was observed, with discordances of 27% (17/63) for ER, 27% (17/63) for PR, and 3% (2/63) for HER2.