The CYP2D6*1xN alleles (*1x2 and *1x鈮?) are assigned as increased function alleles by CPIC. Patients carrying a *1xN allele in combination with an increased or normal function allele or a decreased function allele with an activity value of 0.5 may have a decreased response to ondansetron as compared to patients with alleles that result in a normal metabolizer phenotype. Patients carrying a *1xN allele with an activity value of 3 or greater in combination with a decreased function allele with an activity value of 0.25 or a no function allele may also have a decreased response to ondansetron as compared to patients with alleles that result in a normal metabolizer phenotype, while patients carrying a *1xN allele with an activity value of 2 in combination with a decreased function allele with an activity value of 0.25 or a no function allele may have a similar response to ondansetron as compared to patients with alleles that result in a normal metabolizer phenotype. However, conflicting evidence has been reported. Be aware that the CPIC guideline for ondansetron and CYP2D6 has a 'no recommendation' for CYP2D6 intermediate and poor metabolizers. Other genetic and clinical factors may also influence response to ondansetron.