Tamoxifen, endoxifen, and CYP2D6: the rules for evaluating a predictive factor.

In the post–Human Genome Project era, “personalized medicine” has become a buzzword. Health-care professionals increasingly have access to gene sequence data, with the promise that this information will improve the health of the individual. In the area of breast oncology, the study of genetic markers associated with clinical outcome has been a relative success story. While classic tumor-related factors such as size, grade, and nodal status are still used, researchers have increasingly focused on studying genome-wide genetic variation, both at the level of the tumor and the host, with the goal of identifying subsets of patients at lower or higher risk for disease recurrence. Many of these biomarkers represent classic “prognostic factors,” or markers associated with the biology of the disease.