CXCL13 stimulated prolonged p44/42 MAP kinase activation in CLL B cells. Activation of p44/42 MAP kinase was determined by Western blot after stimulation with 1 μg/mL CXCL13 or 200 ng/mL CXCL12, as indicated on the right-hand side. To determine the time course of p44/42 MAPK activation, CLL cell lysates were obtained at the indicated time points and probed with anti–phospho-p44/42 mAbs. To confirm equal loading of protein, blots were reprobed with an antibody to the nonphosphorylated p44/42 MAPK. Panels A and B display MAP kinase activation in CLL cells from 2 different patients.