<P> The proteins p35 and p39 activate CDK5 . Although they lack cyclin sequence homology, crystal structures show that p35 folds in a similar way as the cyclins . However, activation of CDK5 does not require activation loop phosphorylation . </P> <P> Proteins with no homology to the cyclin family can be direct activators of CDKs . One family of such activators is the RINGO / Speedy family, which was originally discovered in Xenopus . All five members discovered so far directly activate Cdk1 and Cdk2, but the RINGO / Speedy - CDK2 complex recognizes different substrates than cyclin A-CDK2 complex . </P> <P> Leland H. Hartwell, J. Hoonhorst, R. Timothy Hunt, and Paul M. Nurse received the 2001 Nobel Prize in Physiology or Medicine for their complete description of cyclin and cyclin - dependent kinase mechanisms, which are central to the regulation of the cell cycle . </P> <P> CDKs are considered a potential target for anti-cancer medication . If it is possible to selectively interrupt the cell cycle regulation in cancer cells by interfering with CDK action, the cell will die . At present, some CDK inhibitors such as seliciclib are undergoing clinical trials . Although it was originally developed as a potential anti-cancer drug, seliciclib has also proven to induce apoptosis in neutrophil granulocytes, which mediate inflammation . This means that novel drugs for treatment of chronic inflammation diseases such as arthritis and cystic fibrosis could be developed . </P>

How do cyclins and cdks control the cell cycle