Iron and copper reductase activity in HEK-293T cells expressing Steap family members. (A) Ferrireductase activity measured in cells overexpressing Steap family members. (B) Cupric reductase activity in cells overexpressing Steap family members. (C) Ferrireductase activity measured in cells overexpressing Steap family members with only ferric iron added (▪) or ferric iron plus a 5-fold (▦) or 10-fold (□) molar excess of cupric copper. n = 4 for each result in each panel. Error bars represent ± 1 SD.