Analyses of HREBPs in nuclear extracts by gel mobility assays. Gel mobility assays were performed as described in Materials and Methods using nuclear extracts from hemin-treated U937 cells and radiolabeled 27-mer HRE probes. For the experiments in lanes 2 and 4, extracts were preincubated x 5 with beads derivatized with irrelevant (scrambled) or wild-type 27-mer HREs, respectively. Lane 1, free probe. Lane 2, U937 extract preincubated with scrambled 27-mer beads. Lane 3, unmanipulated extract. Lane 4, extract preincubated with beads derivatized with wild-type 27-mer HRE sequence.