Ras guanyl nucleotide-releasing proteins (RasGRPs) are guanyl nucleotide exchange factors (GEFs) that activate Ras ultimately leading to MAPK activation. RasGRPs have a catalytic domain composed of Ras exchange motif (REM) and a CDC25 domain, an atypical pair of EF hands that bind calcium and a DAG-binding C1 domain. After PIP2 hydrolysis, RasGRPs are recruited to the plasma membrane by binding to DAG and calcium (Stone 2011, Liu et al. 2007). Upon T-cell activation RasGRP1 specifically interacts with and activates Ras on Golgi instead of the plasma membrane (Bivona et al. 2003). It remains to be determined whether activation of N-Ras by RasGRP1 in mast cells occurs in the Golgi or the plasma membrane (Liu et al. 2007). RasGRP4 is mast cell specific and is involved in the controls Ras activation.