Gel retardation assay with normal and mutated L ferritin IRE probes. ³²P-labeled L ferritin IRE probe (5 × 10⁴ cpm) of either normal sequence (lanes 1, 2, and 7 to 15) or carrying the loop mutation (mut1, lanes 3 and 4) or the bulge mutation (mut2, lane 5 and 6) was incubated with 5 μg K562 cytoplasmic extracts. The products were analyzed by electrophoresis on nondenaturing polyacrylamide gels either as free probe (lanes 1, 3, and 5) or after incubation with the extract only (lanes 2, 4, and 6). For the competition studies, the normal radiolabeled IRE probe was incubated with the extracts in the presence of a 10-, 50- or 100-fold molar excess of a cold normal IRE probe (lanes 7 to 9) or of a cold mutated IRE probe with a loop mutation (mut1, lanes 10 to 12) or a bulge mutation (mut2, lanes 13 to 15).