DNA that is damaged by ultraviolet (UV) light is repaired predominantly by nucleotide excision-repair, a process requiring the DNA polymerase auxiliary factor PCNA. UV-irradiation also induces the production of Cip1 protein via activation of p53. Cip1 is an inhibitor of the cyclin-dependent kinases, which are required for the cell cycle to proceed through the G1/S-phase transition and initiate DNA replication. Inhibition by Cip1 probably causes the block to initiation of DNA replication that is seen in irradiated cells. Cip1 also directly inhibits the function of PCNA during DNA synthesis. As nucleotide excision-repair requires PCNA, the physiological relevance of PCNA inhibition by Cip1 is currently unclear.