Chicken anemia virus vp3 gene encodes a small protein—Apoptin. It can induce apoptosis in tumorgenic or transformed cells but not in normal cells through a way that is independent of p53 reaction and can not be suppressed by the excessive expression of Bcl-2. All these facts make apoptin a potential antitumor agent. The unique way of apoptin inducing apoptosis is also useful to the study of the mechanisms of cell transformation and pathways of apoptosis.