This successful line of research, started with the identification that flavopiridol, a flavone with antitumor activity, that was the first in class molecule able to inhibit all cell-cycle related cyclin-CDK complexes.39 Although flavopiridol lacks the necessary specificity and potency to become a drug with a possible clinical use, we can say that its identification as CDK inhibitor open a new era in the treatment of several type of human cancers, as better described below (see section “CDKs as therapeutic targets”). Here, PCNA is linked to cancer.