The diverse clinical syndromes associated with hepatitis C underscore the multifactorial and polygenic nature of HCV infection. Both viral and host factors likely contribute to variations in infection outcome, disease susceptibility and progression, and treatment response. This protocol will focus on the immunogenetics of HCV infection. Various candidate genes, most of them related to host immune response in microbial infection, have defined genetic polymorphisms that have been associated with variable manifestations of infections including malaria, tuberculosis, leprosy, AIDS and hepatitis B. In this proposal, we plan to collect peripheral blood mononuclear cells as a source of DNA from approximately 1500 patients with HCV infection, analyze genetic polymorphisms of various candidate genes in association with viral clearance, disease progression or treatment response, and characterize the functional consequences of these polymorphisms in patients with well-defined clinical sequelae of HCV infection. We will also collect blood from patients with other forms of liver diseases (approximately 300) or normal volunteers (approximately 200) as controls. By identifying relevant host factors genetically and investigating their molecular interactions with HCV, we may gain additional insights into HCV pathogenesis and uncover new potential targets for vaccine development and treatment intervention.