Electrophoretic mobility shift assays (EMSA) performed using a radiolabeled probe representing the 21-bp triplication from the FeLV-945 LTR. Nuclear extracts (3.5 μg) from K-562 cells were incubated with the radiolabeled probe (1 ng). Double-stranded competitor oligonucleotides were omitted from the reaction (lanes 0), or were included in increasing amounts from 10-fold to 250-fold molar excess (10-, 25-, 50-, 100- and 250-fold excess shown). The competitors used contained a c-Myb consensus binding site (5'-TACAGGCATAACGG TTCCGTAGTGA) or a CREB consensus binding site (5'-AGAGATTGCCTGACGTCA GAGAGCTAG). Also indicated is the migration of the radiolabeled probe without the addition of nuclear extract (lanes C).