A high sensitive and specific electrochemical sensor based on Exo Ⅲ assisted target cycle and long-range self-assembled DNA nanostructures was developed for the detection of the sequence related to breast cancer.Firstly,a stem-loop hairpin probe was immobilized on the surface of a gold electrode(GE).In the presence of target DNA,it could switch on a cycle including hybridization,digestion and rehybridization as the first signal amplification.Then the addition of two assisted probes could trigger the formation of super sandwich DNA nanostructures on the electrode surface by long-range self-assembly.The above prepared structure could absorb a large number of electrochemically active molecules RuHex on the surface of GE,resulting in an intense electrochemical response as the second signal amplification.Under the optimum conditions,the sensor showed a good linear relationship in the range of 10 amol/L- 10 pmol/L with a detection limit as low as 8 amol/L.Furthermore,the sensor exhibited an ultra-high discrimination ability even against single-base mismatch sequence.All above properties make it a promising candidate for the ultra-low detection of DNA in clinical real samples.