(A). Coupling of signals through modular SH2 and SH3 domains. Signals are initiated via extracellular factors that induce intracellular tyrosine phosphorylation (indicated by the light switch) which are subsequently relayed to downstream targets through SH3 binding partners (indicated by arrowhead). (B). Structure of the Crk family of proteins. The domains are boxed: SH2, Src homology 2; SH3, Src homology 3; Gag, viral group specific antigen; Y221 or Y207, negative regulatory phosphorylation site. The structure of Src is shown at the top of the figure to indicate its spatial arrangements compared to Crk. TK, tyrosine kinase domain.