CYP17A1 and methemoglobinemia: Cytochrome b5 (b5) is familiar to endocrinologists for promoting the 17,20-lyase activity of P450c17 (CYP17A1), thus regulating the synthesis of androgen precursors (8, 9); but the principal action of b5 is the reduction of methemoglobin (10), and b5 deficiency causes DSD with methemoglobinemia (11, 12).