ESR1 and breast cancer: Of 3200 (41.0%) SBC patients with information on the estrogen receptor (ER) status of the PBC and SBC, 571 (18.0%) had an ER–negative first and second breast cancer, 351 (11.0%) had an ER-positive first and ER-negative second cancer, 388 (12.0%) had an ER-negative first and ER-positive second breast cancer, and 1890 (59.0%) had an ER-positive first and second cancer.