Selective serotonin reuptake inhibitors (SSRIs) are the most commonly prescribed medication for major depressive disorder (MDD), yet scientists still do not understand why the treatment does not work in nearly thirty percent of patients with MDD. Now, researchers have discovered differences in growth patterns of neurons of SSRI-resistant patients. The work has implications for depression as well as other psychiatric conditions such as bipolar disorder and schizophrenia that likely also involve abnormalities of the serotonin system in the brain.