(A) RAP1A and RAP1B are GTPases that act as molecular switches to rapidly activate integrins. RAP1 is activated by the guanine exchange factor CalDAG-GEFI. The GTPase activating protein RASA3 serves as a counterbalance to keep RAP1 signaling quiescent in circulating platelets. Deleting both RAP1A and RAP1B identified RAP1-independent mechanisms for integrin activation that are not mediated by RAP2. (B) RAP1A and RAP1B have overlapping and redundant roles in platelets. TxA2, thromboxane A2. Illustration by Xu Han.