The CYP2D6*2 allele is assigned as a normal function allele by CPIC. Patients with breast cancer and carrying the CYP2D6*2 allele in combination with alleles that result in a normal metabolizer phenotype may have decreased likelihood of recurrence and increased event-free and recurrence-free survival when treated with tamoxifen in an adjuvant setting as compared to patients with a no function allele in combination with a decreased or normal function allele or two no or decreased function alleles. However, conflicting evidence has been reported. Be aware that the DPWG guideline for tamoxifen and CYP2D6 has a 'no recommendation' for CYP2D6 ultrarapid metabolizers. Other genetic and clinical factors may also influence response to tamoxifen treatment.