The laboratory evolution of enzymes with tailor-made DNA cleavage specificities would represent new tools for manipulating genomes and may enhance our understanding of sequence-specific DNA recognition by nucleases. Below we describe the development and successful application of an efficient in vivo positive and negative selection system that applies evolutionary pressure either to favor the cleavage of a desired target sequence or to disfavor the cleavage of nontarget sequences. We also applied a previously described in vitro selection method to reveal the comprehensive substrate specificity profile of the wild-type I-Scel homing endonuclease. Together these tools were used to successfully evolve mutant I-Scel homing endonucleases with altered DNA cleavage specificities. The most highly evolved enzyme cleaves the target mutant DNA sequence with a selectivity that is comparable to wild-type I-Scel's preference for its cognate substrate.