Effectiveness Study of Vilazodone to Treat Depression and to Discover Genetic Markers Associated With Response

Inclusion Criteria: * Male or female patients 18-65 years of age, inclusive. * A diagnosis of MDD, single episode or recurrent, according to DSM-IV-TR (296.2/296.3) with a current Major Depressive Episode of less than two year's duration with a minimum duration of at least 4 weeks. * HAM-D score ≥ 22. * HAM-D item 1 (depressed mood) score ≥ 2. * Patients must be able to provide written informed consent to participate before beginning any trial related activities. * Patients must be able to speak, read and understand English and possess the ability to respond to questions and follow simple instructions. Exclusion Criteria: * A current (or within 6 months prior to the Screening Visit) Axis I disorder of Post Traumatic Stress Disorder, Eating Disorder, Obsessive Compulsive Disorder (Generalized Anxiety Disorder, Social Phobia or Simple Phobia will be allowed). * A history of schizophrenia, schizoaffective disorder or bipolar I or II disorder (with a history of hypomanic or manic episodes). * DSM-IV-TR criteria for substance abuse (alcohol or drugs) within 3 months prior to Screening Visit or substance dependence within 6 months prior to the Screening Visit. * Criteria for any of the following DSM-IV-TR MDD Specifiers: \[a\] With Catatonic Features; \[b\] With Postpartum Onset; \[c\] With Seasonal Pattern.