The CYP2D6*5 allele is assigned as a no function allele by CPIC. Patients with breast cancer and carrying the CYP2D6*5 allele in combination with a no, decreased or normal function allele may have increased likelihood of recurrence and lower event-free and recurrence-free survival when treated with tamoxifen in an adjuvant setting as compared to patients with alleles that result in a normal metabolizer phenotype. 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.