ESR1 and breast cancer: In breast cancer (BCa), the GRPR is mainly overexpressed in hormone-driven BCa [12]; the overexpression of GRPRs was found in 83.2% of estrogen receptor (ER)-positive BCa and only in 12% of ER-negative BCa, in 21.3% of human epidermal receptor type 2 (HER2)-positive tumors, and in 7.8% of triple-negative tumors [13].