Mass spectrometry is a powerful tool to identify proteins and post-translational modifications. The upper panel of this figure shows the results of a Maldi profile of tryptic peptides from the breast and ovarian cancer susceptibility protein Brca1. The peaks labelled P1 and P2 are peptides that are phosphorylated by the ATM and ATR kinases. The lower panel shows a MS-MS plot of one of these peptides. This type of analysis allows us to determine the actual sites of phosphorylation.