Figure 5.

Schematic representation of the possible mechanisms of evolution of multidomain protein kinases with more than one homologous but distinct non-kinase domain, as exemplified in the case of the Syk family of cytoplasmic tyrosine kinases. (a) Duplication of the SH2 domain followed by distinct functional specialization of duplicated domains; (b) Recruitment of two homologous but distinct SH2 domains by domain fusion.