Homologous unequal cross-over involving a 2.8 kb direct repeat as a mechanism for the generation of allelic variants of human cytochrome P450 CYP2D6 gene. 
The cytochrome P450 enzyme debrisoquine 4-hydroxylase (CYP2D6) metabolizes many different classes of commonly used drugs.  In Caucasian populations, 5-10% are classified as poor metabolizers (PM) due to autosomal recessive inheritance of two mutant CYP2D6 null alleles.  In contrast, up to 5% may demonstrate ultrarapid metabolism (UM) of debrisoquine caused by inherited amplification of functional CYP2D6 genes in the CYP2D6 locus.  Poor metabolizer subjects may develop toxic plasma concentrations and adverse drug reactions, whereas UMs may suffer from therapeutic failure.  Moreover, mutant CYP2D6 alleles have been implicated as a predictor of susceptibility for diseases such as cancer and neurological disorders.  The break points and molecular mechanisms involved in the generation in the PM-associated CYP2D6(D) gene deletion allele and the UM-related CYP2D6 amplification have not been clarified.  Here we demonstrate the presence of a 2.8 kb repeated region (CYP-REP) which flanks the active CYP2D6 gene in the wild type allele.  The CYP-REP unit may be itself predispose to homologous unequal cross-over and contains the Alu element and a tandem 10 bp direct repeat, which could both serve as hotspots for recombination.  The break points of the CYP2D6(D) deletion allele are present within the repeated 2.8 kb region, but the exact positions are non-determinable due to perfect recombination of the misaligned, homologous CYP-REP elements.  We also propose that the alleles with multiple copies of CYP2D6, which represent the first example of inherited amplification of an active gene in man, can be explained by unequal cross-over events involving the CYP-REP units.  In our model, the CYP2D6 deletion and amplification alleles are reciprocal to each other, generated through homologous unequal recombination of no-allelic CYP-REP elements.