VIRS (A visual tool for identifying restriction sites in multiple DNA sequences) is an interactive web-based program designed for restriction endonuclease cut sites prediction and visualization. It can afford to analyze multiple DNA sequences simultaneously and produce visual restriction maps with several useful options intended for users' customization. These options also perform in-depth analysis of the restriction maps, such as providing virtual electrophoretic result for digested fragments. Different from other analytical tools, VIRS not only displays visual outputs but also provides the detailed properties of restriction endonucleases that are commercially available. All the information of these enzymes is stored in our internal database, which is updated monthly, from the manufacturers' web pages. It is freely available online at http://bis.zju.edu.cn/virs/index.html. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 25: 1525-1527, 2009