Figure 2. The classical and backdoor pathways of adrenal steroidogenesis: The classical pathway is highlighted in blue, and the backdoor pathway is highlighted in orange. In the classical pathway, five enzymatic steps are necessary for cortisol production. In the first step of adrenal steroidogenesis, cholesterol enters mitochondria via a carrier protein called steroidogenic acute regulatory protein (StAR). ACTH stimulates cholesterol cleavage, the first and rate limiting step of adrenal steroidogenesis. The five enzymes required for cortisol production are cholesterol side chain cleavage enzyme (SCC), 17α-hydroxylase, 3β-hydroxysteroid dehydrogenase (3βHSD2), 21-hydroxylase, and 11β-hydroxylase. The backdoor pathway is an alternative pathway producing dihydrotestosterone. The enzymes include 5α-reductase 1, aldo keto reductases, retinol dehydrogenase RoDH, 17β-hydroxysteroid dehydrogenases, 17α-hydroxylase. (Figure from CAH Endotext Chapter).