The DPYD phenotype is assigned using a gene activity score (DPYD-AS; based on the DPYD Allele Functionality Table), calculated as the sum of the two DPYD variants with the lowest variant activity value. Briefly, carriers of two no function variants (DPYD-AS: 0) OR one decreased function variant (DPD-AS: 0.5) are classified as DPYD poor metabolizers; carriers of two decreased function variants OR carriers of only one no function variant (DPYD-AS: 1) OR carriers of only one decreased function variant (DPYD-AS: 1.5) are considered DPYD intermediate metabolizers , and those with only normal function variants are classified as DPYD normal metabolizers (DPYD-AS: 2). If two decreased/no function variants are present, each decreased/no function variant is considered to be on a different gene copy. Irrespective of the presence of decreased/no function variants, patients may carry multiple normal function variants. As an individual only carries a maximum of two fully functional DPYD copies, common normal function variants may be located on the same gene copy as other normal function variants or decreased/no function variants.