CYP2D6 and breast cancer: To our knowledge, so far only one study reported on the effect of CYP2D6 genotype in a small number of Asian breast cancer patients using tamoxifen for metastatic disease evaluating the association of the dysfunctional allele CYP2D6*10/*10 (commonly occurring in the Eastern people in contrast to Caucasians) with clinical outcome (Lim et al, 2007).