The rapid development of molecular and cell biology in the latter part of the last century has led us to the understanding that many diseases, including cancer, are caused by perturbations of cellular networks, which are triggered by genetic changes and/or environmental challenges. These perturbations manifest by changing cellular protein profiles, which, in turn, alter the quantitative relationship of tissue-specific proteins shed into the tissue/organ microenvironment. Such altered protein expression profiles in body fluids constitute molecular signatures or fingerprints that reflect the original perturbation of cellular networks. The exciting challenge of modern proteomics is to identify such signatures for various disease states—then the body fluids will become windows into disease and potential biospecimen sources for biomarkers of disease. (1).