A p53 functional analysis system, which can identify the types of abnormality of p53, such as loss of function, dominant negative function, or gain of oncogenic function, is now required. In this study, we examined the functional diversity of several mutations of p53 derived from human head and neck cancer cells. The entire open reading frame of p53 cDNA was subcloned into a mammalian expression vector, pEGFP-C3, and genetic mutations were determined. Then, intracellular localization and transcriptional activity of the tumor-derived p53 proteins were examined in Saos-2 cells. A mutant-p53 (Glu17Lys, His193Leu) or a truncated p53 (Delta121) did not activate the reporters containing p53 responsive elements from p21waf1, BAX, MDM2, p53AIP1, and PUMA genes at all. However, a mutant-p53 (Asn30Ser) showed the transcriptional activity on all of the reporters as wild-type p53 did. On the other hand, a mutant-p53 (Asp281His) activated the p21waf1 promoter strongly and the MDM2 promoter faintly, but did not activate the BAX promoter. Interestingly, this mutant-p53 prevented Saos-2 cells from undergoing apoptosis after treatment with a DNA damaging agent, adriamycin. This mutant-p53 induced cell cycle arrest but not apoptosis. Furthermore, another mutant-p53 (Glu17Lys, His193Leu) also prevented the cells from undergoing apoptosis after DNA damage probably in a transcription-independent manner. These results suggest that some cancer cells may contain the oncogenic mutation of the p53 gene, and the oncogenic p53 protein prevents cancer cells from undergoing apoptosis after DNA damage. Detailed information for mutated p53 gene in cancer cells might provide useful suggestions for the therapeutic strategy.