CYP21A2 and congenital adrenal hyperplasia: In contrast to 17,20-lyase deficiency in the context of CYP17A1 deficiency (see “CAH due to CYP21A2 deficiency” above), which always comes with a degree of impairment of CYP17A1 17α-hydroxylase activity, CYB5A deficiency represents “true” isolated 17,20-lyase deficiency (15, 70); increased steroid ratios for 17,20-lyase activity with normal ratios for 17α-hydroxylase activity are diagnostic (Table 2).