SOS1 and fibrosarcoma: We analyzed the changes induced by the absence of Sos1 in the activation of two of the main intracellular pathways stimulated by Ras: rapidly accelerated fibrosarcoma (Raf)-mitogen-activated protein kinase (MAPK) cascade, whose final effect is the phosphorylation (on Tyr204) of extracellular signal-regulated kinases (Erk), and the phosphatidylinositol 3-kinases pathway (PI3K) that leads to phosphorylation (on Ser473) of Akt (protein kinase B).