In the study of hepatitis B virus (HBV) and hepatitis C virus (HCV) infection, it was found that cytotoxic CD8+ T cells mediate direct killing of hepatocytes, while CD4+ T cells inhibit virus replication and activates host macrophages by producing IFN-γ and tumor necrosis factor- α (TNF-α) and clear the virus with a non-cytopathic mechanism (21). The gene discussed is TNF; the disease is infection.