Our findings that people with the reduced function CYP2D6*4 haplotype were at a greatly increased risk of all-cause and breast-cancer mortality agree with the Clinical Pharmacogenetics Implementation Consortium (CPIC) Guidelines for CYP2D6 genotyping prior to tamoxifen treatment, which recommend the implementation of CYP2D6 genotyping prior to tamoxifen treatment in breast cancer settings [22]. This evidence concerns the gene CYP2D6 and breast cancer.