In PNAS this week, Steven Gygi at Harvard led work that used stable-isotope dilution and high-resolution mass spec to measure kinase activity. His multiplexed assay was able to measure up to 90 site-specific peptide phosphorylation rates at the same time, and he could apply it to a variety of cells and cellular processes, including mitogen stimulation, cell cycle, pharmacological inhibition of pathways, and to a panel of breast cancer cell lines.