associated with nonclassic 21-hydroxylase deficiency. Due to the high degree of homology between the CYP21A2 gene and the CYP21A1P pseudogene, and the complexity of the locus, research on the molecular level is difficult. Differences in residual enzyme activity of the various alleles account for the various degrees of severity of the disease. Inheritance of all forms of 21-hydroxylase CAH is autosomal recessive. Persons affected by any forms of the disease have two abnormal alleles, and both parents are usually heterozygotes (or carriers). When both parents carry an abnormal allele, each child has a 25% chance of having the disease, a 50% chance of being a carrier like the parents, and a 25% chance of having two normal genes. It is possible to test for heterozygosity by measuring 17α-hydroxyprogesterone elevation after ACTH stimulation. More than 200 disease-causing variants within the CYP21A2 gene have been identified so far that lead to 21-hydroxylase deficiency if at least two of these variants are present as compound heterozygous. There is a good correlation between the genotype and phenotype. As a result, the CYP21A2 genotyping has high diagnostic value. However, the genotyping of the CYP21A2 gene is prone to errors, especially due to the closely located and highly homologous pseudogene CYP21A1P and the complex duplications, deletions and rearrangements within the chromosome 6p21.3. That's why CYP21A2 genotyping, interpretation of the results, and adequate genetic counseling for patients and their families requires deep understanding of CYP21A2 genetics. Pathophysiology The enzyme P450c21, commonly referred to as 21-hydroxylase (21-OH), is embedded in the smooth endoplasmic reticulum of the cells of the adrenal cortex. It catalyzes hydroxylation of 17α-hydroxyprogesterone (17OHP) to 11-deoxycortisol in the glucocorticoid pathway, which starts from pregnenolone and finishes with cortisol. It also catalyzes hydroxylation of progesterone to 11-deoxycorticosterone (DOC) in the mineralocorticoid pathway on its way from pregnenolone to aldosterone. Deficient activity of this enzyme reduces the efficiency of cortisol synthesis, with consequent hyperplasia of the adrenal cortex and elevation of ACTH levels. ACTH stimulates uptake of cholesterol and synthesis of pregnenolone. Steroid precursors up to and including progesterone, 17α-hydroxypregnenolone, and especially 17α-hydroxyprogesterone accumulate in the adrenal cortex and in circulating blood. Blood levels of 17OHP can reach 10-1000 times the normal concentration. Since 21-hydroxylase activity is not involved in synthesis of androgens, a substantial fraction of the large amounts of 17α-hydroxypregnenolone is diverted to synthesis of DHEA, androstenedione, and testosterone beginning in the third month of