Congenital adrenal hyperplasia (CAH) due to CYP21A2 mutations is classified into two major forms: classic CAH, encompassing the salt-wasting and simple-virilizing phenotypes with early-onset cortisol deficiency and androgen excess, and nonclassic CAH, a milder variant with partial 21-hydroxylase activity that typically presents later with hyperandrogenic symptoms. The gene discussed is CYP21A2; the disease is congenital adrenal hyperplasia.