STEAP1 and breast carcinoma: Since its discovery in 1999 as a multispan membrane protein highly expressed on prostate cancer cells (1), six-transmembrane epithelial antigen of the prostate 1 (STEAP1) emerged as a cancer antigen expressed in various human cancers, including prostate, bladder, colorectal, lung, ovarian, and breast carcinoma and Ewing sarcoma.