Specifically, IL-15 crosses the blood–brain barrier (Wu et al., 2010b) and modulation of IL-15/IL-15rα impacts sleep patterns (Kubota et al., 2001), anxiety behavior pathways (Wu et al., 2010a), and anti-depression pathways (Wu et al., 2011), which collectively has profound global impacts. This evidence concerns the gene IL15 and major depressive disorder.