The CYP2E1 gene exhibits copy number variation. Individuals may carry extra copies of CYP2E1. CYP2E1 SNP genotyping assays run on samples having 2 or more gene copies that are homozygous for the SNP allele will cluster together, and samples having more than 2 gene copies that are heterozygous may run between the 2 copy heterozygous and homozygous clusters. For accurate CYP2E1 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.