Two p53-related genes, p73 and p51, were recently identified as structural homologues of the p53 tumor suppressor gene, suggesting that the roles of these two genes may be similar to those of p53, including growth suppression and induction of apoptosis. Here we show that introduction of p73 or p51 cDNAs into cultured human cancer cells suppressed colony formation in the presence of G418. We then examined the ability of various isoforms of p73 and p51 to activate transcription of a reporter gene. This assay showed that p73beta and p51A activated transcription through a consensus p53 binding sequence, while p73alpha and p51B isoforms minimally transactivated the p53 reporter gene. To characterize further the biological functions of the p53-related genes, we constructed recombinant adenoviruses containing the p73 and p51 cDNAs. Ad-p73beta and Ad-p51A induced endogenous p21 gene expression more effectively than Ad-p73alpha and Ad-p51B, respectively. To evaluate the mode of cell death induced by p53-related genes, Ad-p73 and Ad-p51 were used to infect human cancer cells. Infection of Ad-p73beta, Ad-p51A or Ad-p51B resulted in DNA fragmentation in a subset of cancer cell lines more efficiently than did infection of Ad-p53. We then examined the combined effect of each p53-related gene and the E1A oncogene in the induction of apoptosis. The E1A oncogene cooperated with p51 as well as p53 to induce apoptosis, while p73 resulted in a weak induction of apoptosis by E1A. Overall, apoptosis induction by p51B and p73alpha isoforms may be due to mechanisms other than transcriptional activation of p53-target genes. Our results suggest that p53-related genes are both similar to and different from p53 in their pathways leading to growth suppression.