For example, depression is a common comorbidity in breast cancer patients (Massie, 2004) and many selective serotonin reuptake inhibitors (SSRI), which are widely used medications indicated primarily to treat depression (Hansen et al, 2003), are metabolized by CYP2D6 (Zenger et al, 2004). This evidence concerns the gene CYP2D6 and depressive symptom measurement.