MDA-MB-231 cells were transfected with plasmids using Lipofectamine 2000. After 24 hr, cells were washed and collected by scraping into ice-cold PBS, pelleted by centrifugation at 400 × g for 1 min at 4°C, and lysed with 1% Triton buffer for 20 min at 4°C. Crude nuclei and unbroken cells were excluded by centrifugation at 20,000 × g for 5 min at 4°C. As a preclean process, the supernatants were incubated twice with 40 μL Protein G-sepharose beads (GE Health care, 50% slurry in lysis buffer) for 1 hr at 4°C. The supernatants were incubated with 50 μl anti-FLAG M2 affinity gel (Sigma, 50% slurry in lysis buffer) for 16hr at 4°C. The beads were washed with 1% Triton buffer and then three times with wash buffer (50mM Tris-HCl [pH7.5]) to remove detergent, and resuspended in 200 μL digestion buffer (2M urea, 50mM Tris-HCl [pH 7.4], 1mM DTT, 5mM iodoacetamide, 1 μg of trypsin [Promega, V5280]), and incubated by shaking at 1,200 rpm for 16hr at 37°C. The samples were acidified with 22 μL of 10% trifluoroacetic acid (TFA). The supernatant containing peptides were then taken by centrifugation at 4,400 × g for 30 sec. The peptides were desalted by C18Tip (Thermo, StageTips), dried by speedvac, and then resuspended in mass spectrometry buffer (2% acetonitrile, 0.2% TFA).