Proliferating cell nuclear antigen (PCNA) is necessary for DNA synthesis and is an accessory protein for DNA polymerase alpha, which is elevated during the G1/S phase of the cell cycle. PCNA forms a ring around a portion of DNA, serving to anchor the various DNA replication and repair proteins and to regulate proliferation throughout the cell cycle. In staining applications, the PCNA antibody exhibits nuclear staining (1,2). PCNA is overexpressed in many cancer types, and overexpression is correlated with cancer virulence with studies showing that PCNA is directly related to the degree of tumor differentiation, stage of cancer and the prognosis of cancer. PCNA-targeting peptides were shown to inhibit the growth or to induce apoptosis in neuroblastoma, prostate cancer, breast cancer, bladder cancer, and multiple myeloma (3,4).