ATRA/FK228 combination increases binding of the NF-YA to the MDR1 promoter CCAAT box in NB4 cells. NB4 cells were treated with ATRA (1 μM) and/or FK228 (3.0 nM) for 24 hours. Binding complexes were analyzed by gel shift assays using a CCAAT probe. Nuclear extracts from control cells were incubated in the presence of a 30-fold molar excess of unlabeled CCAAT (lane 1) and βRARE oligonucleotides (lane 2) and goat IgG (lane 3). In lanes 4, 6, 8, and 10, no competitors or antibodies were added. The βRARE consensus oligonucleotide does not contain a CCAAT binding sequence. Example of gel shift assay representative of 3 experiments that produced comparable results.