Targeted sequence alteration would be an attractive method in gene therapy and biotechnology. To achieve in vivo targeted sequence alteration, a tailed duplex DNA consisting of annealed 35mer and 794mer single-stranded DNAs was delivered by means of hydrodynamic tail vein injection into liver of transgenic mouse harboring a reporter gene (the rpsL gene) in its genome. The tailed DNA was designed for a conversion of ATC to AGC at codon 80 of the rpsL transgene. The anticipated T → G sequence alteration was induced in the transgene in the liver with an efficiency of ∼0.1%. These results demonstrate the significant potential of this method for applications in gene therapy and biotechnology.