RASA3 (RAS p21 protein activator 3) is a GTPase-activating protein that negatively regulates RAS and RAP1 signaling pathways by enhancing their intrinsic GTPase activity 1. The protein functions as a critical gatekeeper in multiple cell types, maintaining cellular quiescence and controlling adhesion dynamics. In T cells, RASA3 suppresses LFA-1-mediated adhesion and is rapidly downregulated upon activation, with loss leading to increased RAP1 activation and impaired lymph node migration 2. RASA3 serves as a negative regulator of T cell activation and adhesion, helping maintain naive T cells in a quiescent state 3. In endothelial cells, RASA3 controls adhesion turnover and vascular lumen integrity through RAP1-dependent mechanisms, with deletion causing embryonic hemorrhages and lethality 4. The protein also regulates platelet function by restraining RAP1 activation in circulating platelets, preventing inappropriate adhesion while allowing rapid activation upon vascular injury 5. Disease relevance includes associations with pulmonary hypertension, where reduced RASA3 expression correlates with increased mortality and disease severity 6. Additionally, RASA3 shows genetic associations with antibody-mediated immune responses to infectious diseases, particularly Epstein-Barr virus 7. Originally identified as an inositol tetrakisphosphate receptor, RASA3 represents a critical regulatory node controlling cellular adhesion and activation across multiple physiological systems 1.