The purpose of this study is to determine whether pharmacogenetic study predict Mirtazapine responsiveness in advance before the appearance of the drug effect until 4-6 weeks after administration of Mirtazapine.

The purpose of this study is

1. to determine whether genomic differences between drug responders and nonresponders predict the response of Mirtazapine and
2. to construct the prediction model for Mirtazapine treatment in depressed patients in order to aid to select the genetically matching drug.

Inclusion Criteria:

1. Eligible patients were enrolled in the clinical trials program of hte Samsung Medical Center Geropsychiatry and Affective Disorder Clinics(Seoul, Korea). They received a semistructured diagnostic interview, the Samsung Psychiatric Evaluation Schedule. The affective disorder section of the Samsung Psychiatric Evaluation Schedule uses the Korean version of the structured clinical interview for the diagnostic and statistical manual of mental disorders, Fourth edition.
2. interview with one more patient's family member for objective diagnosis and final diagnosis decision by agreements of two more psychiatric physicians

Exclusion Criteria:

1. received psychotropic medication within 2 weeks of the study or fluoxetine within 4 weeks
2. potential study participants for pregnancy, significant medical conditions, abnormal laboratory baseline values, unstable psychiatric features(eg.suicidal), history of alcohol of drug dependence, seizures, head trauma with loss of consciousness, neurological illness, or concomitant Axis I psychiatric disorder.