The small molecule UCN-01 is a cyclin-dependent kinase (CDK) modulator shown to have antiproliferative effects against several in vitro and in vivo cancer models currently being tested in human clinical trials. Although UCN-01 may inhibit several serine-threonine kinases, the exact mechanism by which it promotes cell cycle arrest is still unclear. We have reported previously that UCN-01 promotes G 1 -S cell cycle arrest in a battery of head and neck squamous cancer cell lines. The arrest is accompanied by an increase in both p21 waf1/cip1 and p27 kip1 CDK inhibitors leading to loss in G 1 CDK activity. In this report, we explore the role and the mechanism for the induction of these endogenous CDK inhibitors. We observed that p21 was required for the cell cycle effects of UCN-01, as HCT116 lacking p21 (HCT116 p21 ؊/؊ ) was refractory to the cell cycle effects of UCN-01. Moreover, UCN-01 promoted the accumulation of p21 at the mRNA level in the p53-deficient HaCaT cells without increase in the p21 mRNA half-life, suggesting that UCN-01 induced p21 at the transcriptional level. To study UCN-01 transcriptional activation of p21, we used several p21 waf1/cip1 promoter-driven luciferase reporter plasmids and observed that UCN-01 activated the full-length p21 waf1/cip1 promoter and a construct lacking p53 binding sites. The minimal promoter region required for UCN-01 (from ؊110 bp to the transcription start site) was the same minimal p21 waf1/cip1 promoter region required for Ras enhancement of p21 waf1/cip1 transcription. Neither protein kinase C nor PDK1/AKT pathways were relevant for the induction of p21 by UCN-01. In contrast, the activation of mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK)/extracellular signal-regulated kinase mitogen-activated protein kinase pathways was required for p21 induction as UCN-01 activated this pathway, and genetic or chemical MEK inhibitors blunted p21 accumulation. These results demonstrated for the first time that p21 is required for UCN-01 cell cycle arrest. Moreover, we showed that the accumulation of p21 is transcriptional via activation of the MEK pathway. This novel mechanism, by which UCN-01 exerts its antiproliferative effect, represents a promising strategy to be exploited in future clinical trials.