The loading of proliferating cell nuclear antigen (PCNA) leads to recruitment of pol delta, the process of polymerase switching. Human PCNA is a homotrimer of 36 kDa subunits that form a toroidal structure. The loading of PCNA by RFC is a key event in the transition from the priming mode to the extension mode of DNA synthesis. The processive complex is composed of the pol delta holoenzyme and PCNA (Lee and Hurwitz 1990, Podust et al. 1998). While PCNA increases the processivity of the DNA polymerase delta during telomeric C-strand synthesis in a human telomere replication model, it does not eliminate the DNA polymerase delta stalling on the G-rich template (Lormand et al. 2013).