Inhibition of Bruton's tyrosine kinase (Btk) by PCI-32765. (A) Fluorescent gel scan of splenic lysates from mice that had been treated with PCI-32765 or vehicle. Cells were incubated with the affinity probe PCI-33380, a fluorescently tagged derivative of PCI-32765, prior to lysis and visualization by SDS-PAGE. Binding of PCI-33380 indicates unbound Btk was available in the cells. The arrow indicates the predominant band labeled by the probe (at approximately 76 kDa, the expected molecular weight of Btk). The percentage of occupancy is based on densitometry relative to the vehicle-treated samples. (B) Total Btk expression was determined by western blot.