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