Transgenic mice containing novel oncogene HCCR-2 were generated to analyse the phenotype and to characterize the role of HCCR-2 in cellular events. Mice transgenic for HCCR-2 developed breast cancers and metastasis. The level of p53 in HCCR-2 transgenic mice was elevated in most tissues including breast, brain, heart, lung, liver, stomach, kidney, spleen, and lymph node. We examined whether stabilized p53 is functional in HCCR-2 transgenic mice. Defective induction of p53 responsive genes including p21WAF1, MDM2, and bax indicates that stabilized p53 in HCCR-2 transgenic mice exists in an inactive form. These results suggest that HCCR-2 represents an oncoprotein that is related to breast cancer development and regulation of the p53 tumor suppressor.