The CYP2C6*10 allele has been assigned as a decreased function allele with an activity value of 0.25 by CPIC. Patients carrying the *10 allele in combination with a decreased or no function allele or an increased function allele with an activity value of 2 may have a similar response to ondansetron as compared to patients with alleles that result in a normal metabolizer phenotype, while patients carrying the *10 allele in combination with an increased allele with an activity value of 3 or greater may have a decreased response to ondansetron as compared to patients with alleles that result in a normal metabolizer phenotype. 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.