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Proliferating cell nuclear antigen (PCNA) is a small nuclear protein that plays a central role in DNA replication. Although it has been intensely studied, there are strangely few reports of post-translational modifications to this protein. A site of ubiquitination has been described, and some have claimed the protein is not phosphorylated. A recent publication proposed partial methyl esterification of a number of Glu and Asp residues. Very recently, we found a phosphorylated Tyr (211) and showed it regulates stability of the protein. Increased phosphorylation at this site coincides with pronounced cell proliferation, and also correlates better with poor survival of breast cancer patients than total PCNA level. We describe here the strategy and experimental details of how we found Tyr211 to be phosphorylated.