Classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency, simple virilizing form is a genetic disorder that affects the adrenal glands. It is caused by a mutation in the CYP21A2 gene, which is responsible for producing the enzyme 21-hydroxylase. This enzyme is necessary for the production of cortisol and aldosterone, two hormones that are important for regulating the body's metabolism and stress response. Without enough of these hormones, the body produces too much androgen, a male sex hormone. This can lead to physical changes such as early puberty, abnormal genitalia, and infertility in both males and females.