Elevated Rap1 activity and migration in CD38-expressing CLL cells is linked to intracellular basal Ca²⁺. Cells were depleted of intracellular Ca²⁺, as indicated (Methods). (A) Rap1 activity assays. Representative western blot (left) and quantification (right) of basal active Rap1 in MEC1-GFP and MEC1-CD38H cells. Relative active Rap1 levels were obtained by normalizing each value to the MEC1-GFP control cells. Graph shows the mean of 3 independent experiments ± SEM. *P < .05; **P < .01 determined by 2-tailed Student t test. (B) Migration of MEC1-CD38H cells through transwell filters after calcium depletion. Values represent the percentage of migrated cells divided by the total number of cells added to the filter. Data shown are the mean of 3 independent experiments ± SEM. *P < .05 determined by 2-tailed Student t test. (C) Western blot (top) and quantification (bottom) of basal active Rap1 in primary CLL cells with varying CD38 expression levels (n = 6 CLL patient samples). ns, not significant.