For example, in Adrenal hyperplasia, congenital, due to 21-hydroxylase deficiency (MIM: 201910), it is assumed that the mildest mutation determines the phenotype in compound heterozygotes and that missense variants in CYP21A2 (MIM: 613815) have a less severe effect on enzyme activity than do other variants such as truncation or ablation variants.35 The gene discussed is CYP21A2; the disease is classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency.