ESR1 and breast cancer: In all, 198 HER2-positive BC patients have been enrolled, and the CTC detection rate was 79.8% (158/198): >30% of CTCs were strongly positive for HER2, however just 41.1% (65/158) patients had consistent histology and CTC HER2 status, along with other 58.9% (93/158) patients displaying HER2 positive histology, and CTC HER2 status negative; 98 (62.0%) patients were positive for estrogen receptor (ER) while 75 (47.5%) patients were positive for progesterone receptor (PR); the Ki-67 positive level was higher than 50% in 88 (55.7%) patients.