FIG. 3. Schematic representation of the major steroidogenic pathways. Those enzymes essential in testosterone biosynthesis by testicular Leydig cells are discussed in the text. Cytochrome P450c21 catalyzes 21-hydroxylation in the adrenal gland and is encoded by the CYP21B gene. Cytochrome P450c18 mediates 11beta-hydroxylation and the further reactions involved in the biosynthesis of aldosterone. This enzyme is encoded by the CYP11B2 gene which is expressed only in the adrenal glomerulosa. Cytochrome P450c11 is encoded by the CYP11B1 gene and exhibits exclusively 11beta-hydroxylase activity. Cytochrome P450aro catalyzes the aromatization of triangle4-3-keto C19-steroids into estrogens and is encoded by the CYP19 gene. 5alpha-reductase activity is expressed predominantly in extragonadal tissues and is also discussed in the text.