Proliferating cell nuclear antigen (PCNA) is the component of the chromosomal DNA replication machinery in eukaryotic cells that confers high processivity upon DNA polymerase delta and epsilon. It has been proposed that PCNA functions by forming a trimeric complex with a ring-like structure through which DNA is threaded. PCNA from the yeast Saccharomyces cerevisiae has been crystallized in a cubic space group (P2(1)3, a = 121.1 A). Unexpectedly, a mercury derivative of PCNA yields crystals that diffract significantly better than crystals of the unmodified protein (2.4 A and 3.0 A resolution, respectively). Mass spectrometry reveals that the derivative results from the addition of two mercury atoms to the protein. Although crystals of the mercurated protein show evidence of non-isomorphism, the anomalous diffraction signal is strong and phases may be determined by multi-wavelength anomalous diffraction (MAD phasing).