After multiple cycles of cyclophosphamide treatments, the incidence of liver injury and hematologic toxicities, including lymphopenia, thrombocytopenia and monocytopenia, but not excretory system (kidneys, urinary bladder) toxicities were associated with the patients’ CYP2B6 metabolizer phenotype. The gene discussed is CYP2B6; the disease is Decreased total monocyte count.