This randomized, double-blind, placebo-controlled, multicenter, 8-week, clinical trial is designed to assess the efficacy and safety of vilazodone and to evaluate genetic biomarkers of treatment response associated with vilazodone use in adult patients diagnosed with MDD by the DSM-IV-TR criteria. This study will enroll approximately 470 patients at approximately 10 clinical sites. Safety and efficacy will be assessed at each visit. A DNA sample will be collected and analyzed for response to vilazodone.