The presence of a highly homologous pseudogene, CYP21A1P, forms the basis for the relatively high incidence of 21- hydroxylase deficiency as deleterious sequences can be transferred from CYP21A1P to CYP21A2.
This study demonstrated high allelic variability for 30-kb deletion in patients with 21-hydroxylase deficiency indicating that a founder effect might be improbable for most monomodular alleles carrying CYP21A1P/A2 chimeric genes in Brazil.
Correct diagnosis of 21-hydroxylase deficiency (21OHD) requires the identification of CYP21A2 gene deletions and CYP21A1P/CYP21A2 chimeric genes, which are disease-causing alleles, and gene duplications, which can lead to false-positive 21OHD allele results.
In addition, the haplotype of CYP21 with chimera CYP21P/CYP21 causes 21-hydroxylase deficiency in congenital adrenal hyperplasia (CAH), while chimera TNXA/TNXB is associated with Ehlers-Danols syndrome as well as CAH.
It has been demonstrated that one reaction for PCR amplification of the CYP21 gene and the chimeric CYP21P/CYP21 gene using mixed primers in combination with nested PCR and single-strand conformation polymorphism is considered highly efficient and accurate for molecular diagnosis of CAH due to 21-hydroxylase deficiency.
More than two hundred characterized 21-hydroxylase deficiency alleles appear to result exclusively from sequence exchanges involving the 21-hydroxylase gene (CYP21B) and a closely related pseudogene (CYP21A).
This and most other mutations causing 21-hydroxylase deficiency are normally present in the CYP21P pseudogene and have presumably been transferred to CYP21 by gene conversion.