NRG2 and major depressive disorder: Importantly, the Nrg1, Nrg2, Nrg3, ErbB3 and ErbB4 genes have been identified as susceptibility genes for schizophrenia, depression and bipolar disorder (Mei and Nave, 2014, Rico and Marin, 2011) and numerous genetic and functional studies have directly implicated the Nrg1, Nrg2, Nrg3 and ErbB4 genes in the development of psychotic behaviour (Hayes et al., 2016, O'Tuathaigh et al., 2007, O'Tuathaigh et al., 2010, Shamir et al., 2012, Yan et al., 2017).