Multicellular organisms clearly require mechanisms for intercellular communication and, perhaps even more basically, for intercellular cohesion. The most primitive sponges and coelenterates depend on cell adhesion for their organismal organization; so do insects, nematodes and vertebrates. What molecules and mechanisms are common among these different phyla and which ones differ and why?

The availability of the euchromatic genomic sequences of Drosophila melanogaster (Adams et al., 2000; Rubin et al., 2000; http://www. celera. com) and Caenorhabditis elegans (http://www. sanger. ac. uk/Projects/C_elegans/) makes it possible to address these questions with much more confidence than heretofore. We searched the Drosophila (and, to a lesser extent, the C. elegans) genomic sequences using a large number of vertebrate sequences of adhesion proteins. We also conducted searches for particular domains prevalent in adhesion proteins (Kreis and Vale, 1999) and made extensive use of the listings of Drosophila transcripts sorted by domain family that are available at the EBI web site (http://www. ebi. ac. uk/proteome/). Because of the complex, multi-domain nature of many adhesion proteins (see http://expasy. cbr. nrc. ca/cgi-bin/lists? extradom. txt and Bork and Bairoch [1995] or Kreis and Vale [1999] for listings), significant matches were frequently obtained because of the presence of some shared domains, while other domains were missing. So, homologues were further analyzed for their domain complement and arrangement (using Pfam and Interpro) and for extent of homology by pairwise Blast comparisons. For C. elegans homologues, we referred frequently to the detailed analysis presented by Hutter et al.(2000; http://www. mpimf-heidelberg. mpg. de/ewgdn/). When true orthologues appeared to be absent from flies or worms, we searched extensively with individual domains and with unique segments against the entire genomic sequences. Naturally, all statements about absence of particular genes must be qualified by several cautions. First, the sequence has some gaps and some genes do exist in the heterochromatin, most of which remains unanalyzed. Second, it is always possible that some genes are missed during curation or that distant homologues