FOSB and major depressive disorder: FosB/ΔFosB is a long-half-life transcription factor which accumulates in neurons over days and weeks, and whose expression modulates hippocampal neuron excitability in the context of chronic stress, depression, and addiction (Nestler 2015; Eagle et al. 2018, 2020) and thereby influences the functional phenotype of nearby microglia (Nomaru et al. 2014).