Mutations that cause deficiency of the 21-hydroxylase (21-OH) appear as a result of recombinations between the CYP21B coding gene and the highly homologous CYP21A pseudogene, which are tandemly arranged with the C4A and C4B genes. We report a detailed analysis of a major chromosomic rearrangement by Southern blot using 21-OH and complement C4 cDNA probes, in a wide sample of classic Spanish congenital adrenal hyperplasia (CAH) patients. This study made it possible to observe that 50% of the patients carried at least one allele with gross abnormalities and that the frequencies of alleles with large deletions (16.66%) and gene conversions (14.16%) in the CYP21B gene were very similar. Moreover, our analysis revealed the existence of sixteen different restriction patterns of C4/CYP21 genes. Besides the detection of a new haplotype, which does not seem to appear from unequal crossing-over mechanisms, we observed the highest frequency on CYP21A duplications reported, as well as no duplications of the CYP21B gene. We also observed that although gross abnormalities of the CYP21A pseudogene did not affect 21-OH activity, alleles carrying deleterious point mutations had more rearrangements of the CYP21A gene than normal alleles. Even though the 21-OH deficiency is an autosomic trait, boys in our sample carried 2.6 times more deletions than girls. In contrast, conversion alleles were found equally frequently.

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