Overview of domain families in ADDA. The number of families is given in the last row of each category label. (A) Mobile modules, domain families that co-occur with a variety of different domain families, constitute only a fraction of all domain families. Many domains only occur in single-domain proteins or are always associated with the same domain family (associated families). The majority of domain families contain only a single representative sequence on the 40% similarity level (singletons). (B) Taxonomic distribution of domain families over the three superkingdoms (Archaea, Bacteria and Eukaryota). Left: only associated domain families excluding singletons. Right: only mobile modules. Mobile modules tend to be more widely distributed than associated domains. (C) Annotation of domain families. Left: only associated domain families excluding singletons. Right: only mobile modules. Novel domain families do not overlap with domain families from Pfam, SCOP and InterPro. Mobile modules are well known to curated domain databases, but there are many novel domain families left to be explored.