ESR1 and breast cancer: We included in our study 11 single CTCs and 10 pooled CTC samples matched to 3 tumor biopsies as well as 3 pooled white blood cell (WBC) samples from 3 metastatic BC patients (Supplementary Table 1, Supplementary Fig. 1) representing the 3 major BC subtypes, namely triple-negative BC (TNBC) (patient #1), human epidermal growth factor receptor 2 (HER2)-positive/estrogen receptor (ER)-negative BC (patient #2) and ER-positive/HER2-negative BC (patient #3, luminal BC).