Cyclophosphamide, the cornerstone of breast cancer chemotherapy in Ethiopia, is mainly metabolized to 4-hydroxy-cyclophosphamide by genetically polymorphic CYP3A, CYP2B6, CYP2C9, CYP2C19, and CYP2J2 enzymes (El-Serafi et al., 2015). The gene discussed is CYP2C9; the disease is breast cancer.