CClustal Omega is the latest addition to the Clustal family of general purpose multiple sequence alignment programs for DNA, RNA or proteins. The online version is hosted by EMBL. It offers a significant increase in scalability over previous versions, allowing hundreds of thousands of sequences to be aligned in only a few hours. It will also make use of multiple processors, where present. In addition, the quality of alignments is superior to previous versions, as measured by a range of popular benchmarks. Registration not required.