The CDX2 homeobox transcription factor plays key roles in intestinal development and homeostasis. CDX2 is downregulated during colorectal carcinogenesis, whereas overexpression of CDX2 results in growth inhibition and differentiation of colon carcinoma and intestinal cells. However, the means by which CDX2 functions remain poorly understood. p21/WAF1/CIP1 is one of the cyclin-dependent kinase inhibitors. In addition to its role in cell cycle control, p21 plays critical roles in differentiation and tumor suppression. The overlapping in both the expression and function of CDX2 and p21 in the small intestine and colon strongly suggests a link between these two genes. By means of luciferase reporter and electrophoretic mobility shift assays, we show here that CDX2 transactivated and physically interacted with the promoter of p21 in a p53-independent manner. Moreover, overexpression of CDX2 increased the mRNA expression of p21 in HT-29 colon carcinoma cells, as demonstrated by reverse transcription-polymerase chain reaction. These data suggest that p21 is a transcriptional target of CDX2. Our results may thus provide a new mechanism underlying the functions of CDX2.