Human CYP2D6*9 allele is located in the vicinity of 22q13.1 and is approximately 4 kb in length. This allele, a variant form of the CYP2D6 wild-type allele, encodes cytochrome P450 2D6*9 protein. The CYP2D6*9 allele exhibits a deletion of three nucleotides (g.2613delAGA) at the 3' end of exon 5 that results in a K281 deletion during coding. This alteration in protein sequence decreases the enzymatic activity of the cytochrome P450 2D6*9 protein. [ ]