Pfam is a large collection of protein multiple sequence alignments and profile hidden Markov models. Pfam is available on the World Wide Web in the UK at http://www.sanger.ac.uk/Software/Pfam/, in Sweden at http://www.cgr.ki.se/Pfam/, and in the US at http://pfam.wustl.edu/. The web pages can give access to the alignments, trees, protein structure and other functional information for each family. The Pfam libraries of HMMs can be used locally to define domains in complete genomes. Pfam currently contains over 6,000 protein families and domains.