The human genome encodes at least 10 proteins that bind RAS and activate its intrinsic GTPase activity, resulting in the formation of inactive RAS:GDP and attenuating RAS signaling (reviewed in King et al, 2013). One of the identified RAS GAP proteins is RASA1, also known as p120 GAP. GAP proteins stimulate RAS GTPase activity by inserting a conserved arginine residue into the RAS active site, promoting a conformational change in the active site to allow GTP hydrolysis (Ahamdian et al, 2003; Scheffzek et al, 1997; Ahamdian et al, 1997). In addition to the GAP domain, most RAS GAP proteins also contain membrane targeting domains that facilitate interaction with the plasma membrane where RAS is tethered. In some cases, such as RASA3, membrane localization is constitutive, whereas in others, the GAP proteins are targeted to the membrane in response to cellular signaling. In addition to binding RAS, a number of GAP proteins also mediate other protein-protein interactions and act as scaffolds to integrate signaling; some GAPs are also known to bind and activate other small GTPases such as RAP (reviewed in King et al, 2013). Loss-of-functions mutations in RAS GAP proteins have been identified in a number of cancers (reviewed in Maertens and Cichowski, 2014).