ERBB4 and major depressive disorder: These include schizophrenia-linked genetic polymorphisms of ERBB4 and NRG1 (encoding the ERBB4 ligand, neuregulin-1), deregulation of PIK3CD (encoding PI3Kδ) [15], activation of the dopamine 2 receptor (D2R), which inhibits AKT by activating the AKT phosphatase PP2A [16, 17], and AKT phosphorylation in postmortem studies on the brains of patients with schizophrenia or major depression [18, 19].