STEAP1 and metastatic prostate carcinoma: Six-Transmembrane Epithelial Antigen of the Prostate 1 (STEAP1) is a surface antigen that is expressed in over 80% of metastatic prostate cancer, higher than the 60% positivity on an immunochemistry study conducted at Fred Hutchinson Cancer Center using a H score of 30 as a cutoff for positivity.