1. Objective of project: Interferon-a (IFN)-induced depression is a common complication of its use in treating patients for hepatitis C (HCV), with reports of up to 44% of patients experiencing these depressive side effects. The central hypothesis of the proposed research is that polymorphisms in specific serotonergic genes are associated with a propensity to develop IFN-induced depression. Further, IFN-induced decreases in tryptophan and serotonin levels are putatively related to the emergence of depressive symptoms during IFN therapy. The objective of this proposal is to identify predictors of IFN-induced depression such that depressive side effects can be better managed and treated thus permitting patients to complete a full course of IFN therapy.