An inactive cytochrome P450 CYP2D6 allele containing a deletion and a base substitution.

The cytochrome P450 CYP2D6 is a polymorphic enzyme, for which 5%-10% of Caucasians (poor metabolizers) lack activity. The majority of mutations giving rise to the deficiency have now been identified but some individuals show anomalous phenotype-genotype relationships when screened for the common mutant alleles. We have sequenced all nine exons and intron-exon boundaries in a subject who was phenotypically a poor metabolizer but genotypically heterozygous when screened for the common alleles. A single base-pair deletion (T1795) was detected in exon 3 and a base substitution (G2064A) resulting in an amino acid substitution (G212E) in exon 4. The deletion results in premature termination of translation and a truncated protein. In a group of 50 white Americans, the allele frequency for the new mutant allele was 0.01. The new allele explains some cases of anomalous genotype/phenotype relationships for CYP2D6.