Western blot of UV treated human Jurkat cell lysate showing specific immunolabeling of the ~74k Raf-1 protein (Control). The phosphospecificity of this labeling is shown in the second lane (lambda-phosphatase: I>>-Ptase). The blot is identical to the control except that it was incubated in I>>-Ptase (1200 units for 30 min) before being exposed to the phospho Ser301 Raf-1 antibody. The immunolabeling of Raf-1 is completely eliminated by treatment with I>>-Ptase.