Disclosed is an oligonucleotide structure which can distinguish between a normal cell and an abnormal cell such as a cancer cell and can act only in an abnormal cell. The oligonucleotide structure comprises three oligonucleotide chains, i.e., a trigger chain, an effector chain and a support chain, wherein each of the three oligonucleotide chains is bound to the other two chains so as to form a structure like a three-way joint, and wherein the trigger chain has a site capable of recognizing the sequences of other polynucleotide chains on the outside of a site that is bound to the support chain.
