protein is then analyzed by mass spectrometry and the peptide-mass fingerprint generated can be run through a database to identify the protein. Stable isotope labeling by amino acids in cell culture (SILAC) has emerged as an important method in secretomics – it helps to distinguish between secreted proteins and bovine serum contaminants in cell culture. Supernatant from cells grown in normal medium and cells grown in medium with stable-isotope labeled amino acids is mixed in a 1:1 ratio and analyzed by mass spectrometry. Protein contaminants in the serum will only show one peak because they do not have a labeled equivalent. As an example, the SILAC method has been used successfully to distinguish between proteins secreted by human chondrocytes in culture and serum contaminants. An antibody microarray is a highly sensitive and high-throughput method for protein detection that has recently become part of secretomic analysis. Antibodies, or another type of binder molecule, are fixed onto a solid support and a fluorescently labeled protein mixture is added. Signal intensities are used to identify proteins. Antibody microarrays are extremely versatile – they can be used to analyze the amount of protein in a mixture, different protein isoforms, posttranslational modifications, and the biochemical activity of proteins. In addition, these microarrays are highly sensitive – they can detect single molecules of protein. Antibody microarrays are currently being used mostly to analyze human plasma samples but can also be used for cultured cells and body fluid secretomics, presenting a simple way to look for the presence of many proteins at one time. Implications and significance Discovery of cancer biomarkers Besides being important in normal physiological processes, secreted proteins also have an integral role in tumorigenesis through cell growth, migration, invasion, and angiogenesis, making secretomics an excellent method for the discovery of cancer biomarkers. Using a body fluid or full serum proteomic method to identify biomarkers can be extremely difficult – body fluids are complex and highly variable. Secretomic analysis of cancer cell lines or diseased tissue presents a simpler and more specific alternative for biomarker discovery. The two main biological sources for cancer secretomics are cancer cell line supernatants and proximal biological fluids, the fluids in contact with a tumor. Cancer cell line supernatant is an attractive source of secreted proteins. There are many standardized cell lines available and supernatant is much simpler to analyze than proximal body fluid. But it is unclear whether a