p53 is a tumour-suppressor protein that plays a role in many cellular processes, including regulation of the cell cycle, DNA repair, transcriptional regulation of genes, chromosomal segregation, cell senescence and apoptosis. The protein's role as a transcription factor has shown that deregulated transcription, whether increased or decreased, has the potential to contribute to the formation of human cancers. It was previously reported that binding of two transcription factors, C/EBPβ and RBP-Jκ, to a regulatory site on the p53 promoter regulates its activity, in vitro, in a cell cycle-dependent manner. C/EBPβ is a CCAAT enhancer-binding protein that is a member of the basic leucine zipper transcription factor (bZIP) family that plays an important role in mediating cell proliferation, differentiation and can also be involved in inflammatory responses, metabolism, cellular transformation, oncogene-induced senescence and tumorigenesis. RBP-Jκ participates in the transcriptional regulation of target genes by interacting with the cytoplasmic domain of the Notch receptors. When RBP-Jκ is released, transcriptional repression of its target genes occurs through the recruitment of co-repressor complexes and prevents transcription from occurring. Our reports, here and previously published, show that repression of p53 by RBP-Jκ and activation of p53 by C/EBPβ through differential binding of these two factors indicates a type of co-operative regulation in p53 expression. Here, we demonstrate through the use of chromatin immunoprecipitation (ChIP) assays that the co-ordinated binding of these two factors to the p53 promoter occurs in vivo and serves to regulate p53's activity during the cell cycle.