How Vav is activated in RP105 signaling is unclear, but the scenario that Lyn phosphorylates Vav, thereby enhancing its GEF activity, could be envisaged by the following: (1) activation of Lyn by RP105 stimulation; and (2) similar downstream defects in RP105 signaling between Lyn^–/– and Vav1^–/–Vav2^–/– B cells.⁵