Grb2 is widely expressed and is essential for multiple cellular functions. Inhibition of Grb2 function impairs developmental processes in various organisms and blocks transformation and proliferation of various cell types. It is thus not surprising that targeted gene disruption of Grb2 in mice is lethal at an early embryonic stage. Grb2 is best known for its ability to link the epidermal growth factor receptor tyrosine kinase to the activation of Ras and its downstream kinases, ERK1,2.  Grb2 is composed of an SH2 domain flanked on each side by an SH3 domain. Grb2 has two closely related proteins with similar domain organizations, Gads and Grap.  Gads and Grap are expressed specifically in hematopoietic cells and function in the coordination of tyrosine kinase mediated signal transduction.