Figure 1

Domain architectures of the human signalling proteinsVav, Grb2, Nck1 and Crk, as predicted using SMART [14,15]. Although these proteins share two common domains (namely Src-homology 2 and 3, or SH2 and SH3, domains) they are not all homologous to one another. Rather, their SH2 domains are homologous and their SH3 domains are homologous. The complex architectures of these proteinsimply that the co-occurrence of these two domain types has arisen on more than one occasion, and thus that the domain combinations seen in the four proteins are not likely to have arisen from a common ancestor with whom they share a common architecture.