The function of p53 as a tumor suppressor is mainly due to its activity as a transcription factor, activating many genes in response to genotoxic stress. This activity may be the basis for its ability to protect cells against DNA damage leading to growth arrest or apoptosis. The profile of gene expression induced by p53 is heterogeneous, varying across cell lines and tissues, and it is likely that some of the p53-induced genes may be secondary to its primary role as a transcription factor. The principal targets for p53 may themselves be of great value as tumor inhibitors, as was demonstrated for p21waf and MDM2. In order to distinguish between primary and secondary targets for p53 regulation, we analyzed the profile of gene expression regulated by p53 in the presence and absence of inhibitors of protein synthesis, using oligonucleotide microarrays containing probes for approximately 7,000 human genes. Only 25% of the p53-induced genes are also induced in the absence of protein synthesis and may be considered primary targets. We used cluster analysis of these data to classify the p53-regulated genes into early and late genes according to the kinetics of their expression. The results indicate that p53 simultaneously regulates a plethora of genes involved in many cellular functions, such as DNA repair, cytoskeleton and extracellular matrix repair, signal transduction and oxidoreduction as well as growth arrest and apoptosis.