BDNF Gene Polymorphism and Antidepressants 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.