20 × 106 CD8+ T cells per immunoprecipitation were fixed by adding a 1/10th volume of fixation solution (11.1% formaldehyde, 100 mM NaCl, 1 mM EDTA, 0.5 mM EGTA, 50 mM Hepes) to 1 volume of culture media and were incubated for 10 or 30 min at RT.