Our initial venture into the realm of proteomics was led by Wei Yan in collaboration with Ruedi Aebersold at the Institute of Systems Biology. We began by using tandem mass spectrometry to analyze the global proteome of the Huh7 hepatoma cell line and cultured primary and immortalized human fetal hepatocytes [18] . We also used isotope coded affinity tag (ICAT) technology to measure the changes in protein abundance that occurred in human hepatocytes in response to interferon treatment [19] . These analyses led to the generation of a liver proteome dataset consisting of 2,159 unique proteins. Among the identified proteins were 78 interferon-regulated proteins that play roles in a multitude of cellular functions including antiviral defense, immune response, cell metabolism, and signal transduction. These data also contributed to the development of PeptideAtlas, a public resource for further annotating and validating the human genome by mapping identified peptide sequences to the human genome sequence [20] .