Ras-related Protein 1B (Rap1B) is a membrane-localized member of the Ras small GTPase superfamily of proteins. It is a 184 amino acid (aa) propeptide that is processed into a mature 181 aa protein with a predicted molecular mass of 21 kDa. The rat and mouse Rap1B orthologs share 99% and 100% aa identity with the human protein, respectively. Multiple human Rap1B splice variants have been identified. The enzymatic activity of Rap1B is activated by RAP1GAP or EPAC2. Rap1B has been shown to participate in the organization of the vascular lumen by regulating endothelial cell polarity. It also plays a significant role in platelet function, where it mediates adhesion signaling.