PROSITE is an annotated collection of biologically meaningful motif descriptors dedicated to the identification of protein families and domains. Its current release 18.43 (January 4, 2004) contains 1311 documentation entries describing 1775 motif descriptors, which are either patterns or profiles (Hulo et al, 2004; Sigrist et al, 2002). Both are derived from multiple alignments of homologous sequences, giving them the notable advantage of identifying distant relationships between sequences that would have passed unnoticed based solely on pairwise sequence alignment. PROSITE is also a member database of InterPro (Mulder et al, 2003) and a main contributor to the InterPro annotation. More information on PROSITE is available at http://www.expasy.org/prosite.