FOS and major depressive disorder: Cluster #0 (lateral habenula), Cluster #6 (depression), Cluster #8 (nucleus accumbens), Cluster #9 (c-Fos), Cluster #10 (nicotine), Cluster #16 (zebrafish), and Cluster #18 (GCaMP) had the most recent citation bursts and so are indicative of current research fronts.