XRCC6 and B-cell chronic lymphocytic leukemia: Analyses of post-translational modifications (PTMs) on the Ku heterodimer by high-resolution 2D-gel electrophoresis and the mass spectrometry of chronic lymphocytic leukemia (CLL) proteins allowed the identification of the phosphorylation of Ser27 and Ser33 of Ku70 in the resistant form of CLL [138].