Immunofluorescence staining

Cells were harvested, suspended in 100µL PBS with 2% FBS and cytospun onto glass slides using a Shandon Cytospin 4. Cells were then fixed and slides were processed and stained as previously described [38] using primary antibody. Cells were examined for foci and images were acquired using a Nikon fluorescent microscope Eclipse 80i (100 × /1.4 oil, Melville, NY) with a CCD (charge-coupled device) camera and the imaging software NIS Elements (BR 3.00, Nikon). 100 cells per slide were examined for foci formation and considered positive at > 20 foci. For antibodies used, see Supplemental Table 1.