P2RX7 and major depressive disorder: Variation in the P2RX7 gene, encoding the P2X7 receptor, located on 12q24.31, has been implicated in both bipolar disorder (BD) and major depressive disorder (MDD) [20], but the majority of studies so far focused on a single SNP, rs2230912 [21–23], where the minor G allele is associated with a gain-of-function phenotype for increased IL-1β release in monocytes [24] and possibly also in microglia, leading to neuroinflammation and increased risk for mood disorders [23,25].