More than 90 percent of cases result from 21-hydroxylase deficiency (21OHD) caused by mutations in CYP21A2. The prevalence of 21OHD is estimated to be 1:15,000–16,000 in the USA and Europe [3] and slightly lower in Japan (1:18,000) [4,5,6]. Here, CYP21A2 is linked to classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency.