(A) Left: FCS data for the dimerization of GTP-bound K-Ras4B (GTP) and GTP-bound K-Ras4B with the RBD-LeuZ crosslinker (GTP + crosslinker) on supported lipid bilayers, and the estimated dissociation constants for each. The lack of change in diffusion as a function of surface density without the crosslinker suggests that K-Ras4B does not dimerize on its own. Right: step size distributions of K-Ras4B shows similar results: without the crosslinker, there is no change in diffusion as the surface density is increased. (B) FCS measurements for the wild type Btk PH-TH domains (left) and the mutant lacking key dimerization residues (right). Substantial dimerization is only observed for the wild type at a high PIP3 surface density.