5. Clinical relevance: There are currently no known, reliable predictors of IFN-induced depression. The chronic disease of HCV infection collectively affects approximately 4 million Americans and 200 million people worldwide. IFN is the only clinically approved medication whose long-term use can reduce the risk of a fatal outcome and even be curative in some individuals. However, side effects associated with IFN therapy represent a major obstacle to adequate treatment for patients with HCV, often resulting in the discontinuation IFN therapy.