21-Hydroxylase deficiency (21-OHD; MIM #201910), the most common form of CAH, arises from mutations in the <i>CYP21A2</i> gene, resulting in virilization of the external genitalia in affected females, early puberty in males, and short stature. This evidence concerns the gene CYP21A2 and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency.