Just knowing that a protein is expressed in a cell does not reveal what it is up to; increasingly, the chemical modifications it undergoes are the key to understanding its function at any given time. A study from the laboratory of Steven Gygi, Harvard Medical School assistant professor of cell biology, uses one of the most powerful tools of proteomics, mass spectrometry, to pinpoint the dynamics of a common alteration in cells: protein phosphorylation, or the addition of phosphate groups. In order to track the changes, Gygi’s team bathed cells in a brew that contained two amino acids made from the stable isotope carbon-13 instead of standard carbon-12. Proteins in these cells were slightly heavier, so they could be distinguished from normal proteins in a mass spectrometer. By exposing normal cells to a stimulus and comparing the protein changes to labeled cells, the researchers could quantify how the stimulus affected phosphorylation of the proteins. Published in the Jan. 18, 2005, Proceedings of the National Academy of Sciences, the study demonstrates the accuracy of this approach and reveals new information about a recently discovered protein, TSC2, or tuberin, a molecule implicated in cancer.