Activation of Ras is key for lymphocyte development and function. The understanding of Ras activation in T cells now takes an important step forward by the initial analysis of mice lacking the Ras activator RasGRP. tion of GDP from Ras to allow its association with GTP. One mammalian GEF, homologous to the Drosophila gene Son of Sevenless (Sos), forms a complex through its polyproline-rich sequences with the SH3 domains of the adaptor Grb2. Stimulation of the T cell receptor (TCR) drives the Grb2-Sos complex to the membrane, via a phosphoprotein termed LAT (linker for activated T cells). The SH2 domain of Grb2 binds to particular phosphorylated tyrosine residues in LAT (Fig. 1). Targeting of only the catalytic subunit of Sos to the membrane of lymphocytes is sufficient to induce GTP-bound Ras arguing that this GEF might be the connection between antigen receptors and Ras3. Mystery solved? Apparently not, because recent analyses of antigen receptor signaling have indicated additional complexity in Ras activation. Moreover, the Grb2-Sos mechanism could not explain the striking activation of Ras in lymphocytes after phorbol ester stimulation.