MUMmer is a system for rapidly aligning entire genomes, whether in complete or draft form. For example, MUMmer 3.0 can find all 20-basepair or longer exact matches between a pair of 5-megabase genomes in 13.7 seconds, using 78 MB of memory, on a 2.4 GHz Linux desktop computer. MUMmer can also align incomplete genomes; it can easily handle the 100s or 1000s of contigs from a shotgun sequencing project, and will align them to another set of contigs or a genome using the NUCmer program included with the system. If the species are too divergent for a DNA sequence alignment to detect similarity, then the PROmer program can generate alignments based upon the six-frame translations of both input sequences. The original MUMmer system, version 1.0, is described in our 1999 Nucleic Acids Research paper. Version 2.1 appeared a few years later and is described in our 2002 Nucleic Acids Research paper, while MUMmer 3.0 was recently described in our 2004 Genome Biology paper. We have also developed a GPU accelerated version of MUMmer called MUMmerGPU. Registration not required.