STAT3 and B-cell chronic lymphocytic leukemia: First, evidence for the Mt localization of STAT3 has comefrom observations that GRIM-19, a component of the ETC complex I, directly binds toSTAT3.12 Using an antibody that captures all components ofcomplex I and immunoprecipation assays, we co-precipitated serine phosphorylated STAT3 in CLL-BC,but not in N-BC extracts (Figure 4b).