IFN-α-associated depression develops in a total of 30–70% of HCV-infected patients during treatment, in which mild to moderate depression accounts for 45–60%, moderate to severe depression, 15–40%, and major depression, 15–45% (Raison et al., 2005a,b; Schaefer et al., 2002, 2003, 2007, 2012). The gene discussed is IFNA2; the disease is depressive disorder.