Interleukin (IL)-2, a T-helper 1 (Th1) cell-derived cytokine, which potently modulates dopamine activity and neuronal excitability in mesolimbic structures, is linked with pathological outcomes (e.g., schizophrenia, depression, etc.)that at least partly reflect alterations in central dopaminergic processes. The gene discussed is IL2; the disease is major depressive disorder.