The CYP2D6 gene exhibits copy number variation. Individuals may carry deletion alleles or extra copies of CYP2D6. CYP2D6 SNP genotyping assays run on samples lacking CYP2D6 genes will not amplify, homozygous samples having 1 or more gene copies typically cluster together, and heterozygous samples with more than 2 copies may run between the 2 copy heterozygous and homozygous genotype clusters. In addition, some CYP2D6 alleles contain CYP2D7 pseudogene sequences. For accurate CYP2D6 genotype analysis, copy number analysis must be done. For more information, refer to the PGx Experiments User Guide (Pub. # MAN0009612) Chapter 2 Copy Number Variation section.