Rap1 (Krev-1/smg p21), a small-molecular-weight GTP-binding protein that belongs to the Ras-like superfamily of GTPases, is involved in signal transduction cascades. It is highly homologous to Ras but it is down regulated by its own set of GAPs (GTPase-Activating Proteins). Rap1 is implicated in the regulation of a variety of cellular processes including the control of platelet activation, T-cell anergy, B-Cell activation, and neuronal differentiation. Very recently, Rap was shown to be involved in the control of cell adhesion, in particular the regulation of integrin activation by inside-out signaling. In humans, four isoforms of Rap, Rap1A, Rap1B, Rap2A, and Rap2B, exist. Rap1A and Rap1B share more than 90% sequence identity. Rap1A/B is the closest relative of the Ras protein with more than 50% sequence [...]