Cip1 is a 21 kD protein that interacts with and inhibits cyclin-dependent kinases (cdks). Expression of Cip1 is induced by the tumour suppressor p53, and tumour cells have greatly reduced levels of Cip1. As cdks are required for normal progression through the cell cycle, their inhibition by Cip1 may mediate the ability of p53 to block cell proliferation. Cip1 has also been shown to inhibit the DNA polymerase delta auxiliary factor PCNA (proliferating cell nuclear antigen), which is required for replication-fork elongation, and this could be an alternative mechanism by which p53-induced Cip1 blocks cell proliferation.
Strausfeld, U. P., Howell, M., Rempel, R., Maller, J. L., Hunt, T., & Blow, J. J. (1994). Cip1 blocks the initiation of DNA replication in Xenopus extracts by inhibition of cyclin-dependent kinases. Current Biology, 4(10), 876-883. https://doi.org/10.1016/S0960-9822(00)00196-2