Membrane-bound sphingosine (SPG) in cells attenuates basal Ras activity by stimulating the activity of Ras GTPase-activating proteins (RasGAPs). Upon its phosphorylation by SPHK1, SPG is converted to sphingosine 1-phosphate (S1P) which then displaces from GAP downregulating RASA1 (p120GAP) activity and thereby induces Ras-GTP accumulation. This overall increases the level of activated Ras-GTP leading to activation of the ERK/mitogen-activated protein kinase (MAPK) pathway and cell division (Shu et al. 2002, Wu et al. 2003, Spiegel & Milstien 2006).