Binding of the LXRα:PPARα heterodimer to Site I is required to express its repressor function in McArdle RH7777 cells. A mutant derivative of the human CYP7A1 gene promoter–reporter gene chimera (mut.hCYP7A1.CAT, 1 µg) carrying the multibase substitution at Site I was transfected into hepatoma cells along with PPARα (0.5 µg), LXRα (0.5 µg) or RXR (0.5 µg) expression plasmids. The cells were treated with WY 14,643 and 25-HC, where indicated, in medium containing 5% lipoprotein-deficient serum. The promoter activities are expressed relative to the activity (100%) of the wild-type human CYP7A1 gene promoter.