p73 transcription factors are members of the p53 family and participate in developmental processes and DNA damage response. p73 expression was shown to be regulated during the cell cycle, suggesting that p73 might play a role in cell growth and might be a target for cyclin-dependent kinases. Consistent with this hypothesis, we discovered that p73 interacts physically with various cyclins (A, B, D, and E). Furthermore, cyclin A/CDK1/2, cyclin B/CDK1/2, and cyclin E/CDK2 complexes can phosphorylate multiple p73 isoforms in vitro at threonine 86. A specific antibody directed against phosphorylated Thr86 showed that this site is phosphorylated in vivo and that such phosphorylation is regulated in a cell cycle-dependent manner. Thr86 phosphorylation is induced during S phase and is maximal in the G2/M phase. Accordingly inhibitors of cell growth, such as p16 and serum starvation, reduce Thr86 phosphorylation. Finally, we found that cyclin-dependent kinase (CDK)-dependent Thr86 phosphorylation represses the ability of p73 to induce endogenous p21 expression. Our results demonstrate that p73 proteins are targets of CDK complexes and that phosphorylation on Thr86 by CDKs regulates p73 functions.