21-hydroxylase-deficient congenital adrenal hyperplasia is a disease caused by the mutation in CYP21A2 gene, which results in the loss or deficiency of the 21-hydroxylase enzyme, a key enzyme involved in the synthesis of cortisol, and then resulting in reducing cortisol synthesis and increasing testosterone synthesis, and thereby leading to the clinical manifestations of acute adrenal failure, virilisation in girls, and early puberty in boys.