Tamura K, Imamura CK, Takano T, et al. - Researchers conducted this randomized, open-label, multicenter, phase 2 study evaluating the impact of CYP2D6 genotype-guided tamoxifen dosing on the clinical outcome in patients with hormone receptor-positive metastatic breast cancer. In this study, enrollment of 186 patients who needed first-line tamoxifen therapy was done in Japan between December 2012 and July 2016. Based on individual CYP2D6 genotype, they administered tamoxifen at an increased dose (40 mg daily) or regular dose (20 mg daily) to patients heterozygous (wild type [wt]/variant [V]) or homozygous (V/V) for variant alleles of decreased or no function, and tamoxifen at 20 mg daily was provided to patients homozygous for wild-type alleles (wt/wt). Outcomes revealed that increasing tamoxifen dosing did not lead to a higher progression-free survival rate at 6 months in patients with CYP2D6-variant alleles. Individual variability in the efficacy of tamoxifen cannot be solely explained with the CYP2D6 genotype.