We evaluated mature BDNF (mBDNF), S100B levels in plasma and their associations with depression severity in 30 Selective Serotonin Reuptake Inhibitor (SSRI)-resistant MDD patients enrolled in a randomized controlled trial of ketamine compared (n = 20) to a placebo (n = 10) control (saline). This evidence concerns the gene S100B and depressive symptom measurement.