Proliferating cell nuclear antigen (PCNA) is a 36 kDa homotrimeric protein known to act as a co-factor for DNA polymerase δ, which is responsible for leading strand DNA replication. PCNA was originally identified as an antigen that is expressed in the nuclei of cells during the DNA synthesis phase of the cell cycle. A cell cycle-dependent protein called cyclin was shown to be identical to PCNA. Crystal structure data suggests that a PCNA homotrimer ring can encircle and slide along the DNA double helix. Multiple proteins involved in DNA replication, DNA repair, and cell cycle control bind to PCNA rather than directly associating with DNA, thus facilitating fast processing of DNA. PCNA is a useful marker for DNA synthesis and is highly conserved among most species. Human PCNA was over-expressed in E. coli as a recombinant full-size protein without any tag and highly purified.