The purpose of this study is to determine whether alteration of signal transduction components after antidepressant targeting, predict antidepressant responsiveness in advance before the appearance of the drug effects until 4~6 weeks after drug administration, or represent the clinical status of depressed patients.

The purpose of this study is to determine whether antidepressants effect on signal transduction in peripheral lymphocyte and to determine whether the signaling differences between drug responders and nonresponders predict the antidepressant response or represent the clinical status of depressed patients.

Inclusion Criteria:

1. met the Diagnostic and Statistical Manual of the American Psychiatric Association (DSM-IV) criteria for MDD from the Department of Psychiatry, Sungkyunkwan University, Seoul, South Korea.
2. were 18 years of age or older, the existence of a unipolar major depressive episode as verified by DSM-III/IV criteria, at least 2 years after first episode onset, and agreement to informed consents

Exclusion Criteria:

• pregnancy
• significant medical conditions
• abnormal laboratory baseline values
• unstable psychiatric features (e.g, suicidal attempt)
• history of alcohol or drug dependence, seizure, neurological illness, or concomitant Axis I psychiatric disorder