Selective serotonin reuptake inhibitors (SSRIs) are the most commonly prescribed antidepressants, but they don't work for everyone. Now researchers have discovered a gene that may reveal whether people are likely to respond well to SSRI antidepressants, both generally and in specific formulations. The new biomarker, once validated in clinical trials, could be used to create a genetic test, allowing doctors to provide personalized treatment for depression.