HCV research and the NIDA P30 Center also provided our entry point into proteomics. The ability to look at the protein content of a cell provides a strong complement to our genomic analyses, allowing us to evaluate how gene expression changes in response to virus infection translate into changes in the abundance of proteins, the molecules that directly carry out biological functions. Moreover, proteomic analysis of body fluids, such as blood, holds the promise of identifying candidate biomarkers for human diagnostic or prognostic applications.