ESR1 and breast cancer: Hence, the objectives of this study were to compare estrogen receptor (ER), progesterone receptor (PR), and HER2 expression in three tumor compartments (primary tumors, CTCs, and metastases) in patients with breast cancer and to evaluate TGF-β type 1 receptor (TGF-β RI) expression in CTCs as a prognostic factor correlated with the progression-free and overall survival of these patients.