Western blot using ATR polyclonal antibody (Cat # PAB9924) in HeLa cell nuclear extract (Lane 1). Lane 2 shows negligible staining after pre-incubation of antibody with the immunizing peptide (50 ug peptide for 1 h at room temperature followed by centrifugation). A 4-8% gradient gel was used for separation. Goat serum was used at 5% for blocking. The arrowhead corresponds to 301 kDa ATR. The primary antibody was used at a 1 : 1,400 dilution.