ERBB4 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 (16, 17) and numerous genetic and functional studies have directly implicated the Nrg1, Nrg2, Nrg3, and ErbB4 genes in the development of psychotic behavior (18–22).