Analyses. Using linear regression, genotype will be the independent variable, and co-varying for baseline BDI-II score, the maximal BDI-II score will be examined as a dependent variable. An ANOVA for repeated measures will be performed to determine the effects of IFN therapy on tryptophan, 5-HT, and cortisol levels. In addition, the relationship between interferon induced MDD and certain polymorphisms will be examined.