Colon cancer is the second leading cause of cancer death in the United States. Krüppel-like factor 4 (KLF4) is a transcription factor involved in both proliferation and differentiation in the colon. It is down-regulated in both mouse and human colonic adenomas and has been implicated as a tumor suppressor in the gut, whereas in breast cancer, KLF4 is an oncogene. KLF4 is also involved in reprogramming differentiated cells into pluripotent stem cells. KLF4 can act as a transcriptional activator or repressor, but the underlying mechanisms are poorly understood. We found that p300, a CREB-binding protein-related protein, interacts with KLF4 both in vitro and in vivo and activates transcription. We further made the novel observation that endogenous KLF4 is acetylated by p300/CBP in vivo and that mutations of the acetylated lysines resulted in a decreased ability of KLF4 to activate target genes, suggesting that acetylation is important for KLF4-mediated transactivation. Furthermore, we found that KLF4 differentially modulates histone H4 acetylation at the promoters of target genes. Co-transfection of KLF4 and HDAC3 resulted in a synergistic repression of a cyclin B(1) reporter construct. Our results suggest that KLF4 might function as an activator or repressor of transcription depending on whether it interacts with co-activators such as p300 and CREB-binding protein or co-repressors such as HDAC3.