sentence: Ras GTPases activation is promoted by GEFs such as SOS , RasGRF , RasGRP , and RalGDS , whereas Ras GTPases are inactivated by GAPs such as RASA 1 – 3 , RASAL 1 – 3 , DAB2IP , NF1.SPRED 1 – 3 , and SYNGAP1 [ 19 ] .