This portal provides access to an extensive and growing database of human phosphorylation sites and corresponding phosphopeptides that have been observed across hundreds of mass spectrometry-based phosphoproteomics discovery experiments. In addition to cataloging these sites we provide a tool for the rapid design and implementation of targeted phosphoproteomic assays. You can mix-and-match phosphosites from pre-assembled lists and/or select the phosphopeptides you are interested in.