All CAH forms are characterized by an autosomal recessive inheritance. The genes that encode the different enzymes have all been elucidated (1). 21-Hydroxylase is encoded by the CYP21 gene. CYP21 and the homologous pseudogene CYP21P are located in the human leukocyte antigens (HLA) major histocompatibility complex on chromosome 6. CYP21 and CYP21P are 98% identical, which explains the high mutation rate of the CYP21 gene. One-third of the described mutations are deletions and large gene conversions that were caused by recombinations between the two homologous loci. The other mutations are small defects that are transferred from the CYP21P pseudogene to the active CYP21 gene, resulting from nonhomologous pairing. The genotype-phenotype correlation is reported to be 80-90%, with the best predictions in the most severe and the mildest deficiencies (1).

Of all types of congenital adrenal hyperplasia, 21-hydroxylase deficiency is the most frequent and the most extensively studied. Therefore, this chapter on male hypogonadism in CAH predominantly focuses on 21-hydroxylase deficiency.

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