The main hypothesis is that the therapeutic response and pharmacological resistance to ADs in depressed patients can be associated with a polymorphism for the BDNF gene. The research of allelic forms associated to lesser efficiency or inefficiency of ADs could add to the body of evidence that BDNF mediates the mechanism of action of ADs, and could have important practical implications. We propose to compare in a group of patients with major depression, the allelic variability of the BDNF gene between responders and non-responders after a 3-week period and a 6-week period of SSRI treatment.

• The main objective is to research an association between polymorphism for the BDNF gene and the 3-week and 6-week response to a SSRI treatment (escitalopram) in major depression.
• A case control pilot study without any direct individual benefit (200 patients).
• Study period: 24 months.
• Scores from reliable and validated psychometric scales (Montgomery and Asberg Depression Scale, Mini International Neuropsychiatric Interview) specifically analysing depression dimension at the time of inclusion, after three and six weeks of antidepressant treatment.
• Genetic analysis researching a polymorphism of the BDNF gene is done by a blood sample at the baseline.

Inclusion Criteria:

• Patient suffering a severe depressive episode (according to DSM-IV), evolving since at least 2 weeks before entering the study
• Age superior to 18 years
• Caucasian type
• In absence of any medicinal treatment that could enhance depression: methyldopa, beta-blockers, reserpine,
• In absence of hypothyroidia or anaemia
• Without comorbidity that could affect therapy response : e.g. food disorder, substance abuse or dependence syndrome

Exclusion Criteria:

- Will be excluded from the study the patients for which a modification of the depression diagnosis (according to the DSM-IV criteria) takes place during the period of study