An estimated 30% to 40% of patients with depression do not sufficiently respond to treatment with selective serotonin reuptake inhibitors (SSRIs) and the period in which treatment efficacy can be assessed is relatively long. Therefore, a test to identify potential nonresponders could be useful in the treatment of depression. Serotonin transporter gene (SLC6A4) variations have been reported to account for differences in the way individuals respond to SSRI treatment.