The MUMmer sequence alignment package is a suite of computer programs designed to detect regions of homology in long biological sequences. Version 2.1 makes several improvements to the package, including: increased speed and reduced memory requirements; the ability to handle both protein and DNA sequences; the ability to handle multiple sequence fragments; and new algorithms for clustering together basic matches. The system is particularly efficient at comparing highly similar sequences, such as alternative versions of fragment assemblies or closely related strains of the same bacterium.