The next thing he had to do is show that this receptor can bind to cognate response elements in the promoter for CYP3A, and he did that by doing a gel study, an electrophoretic mobility shift assay, where you ask whether a particular protein can stick to a small piece of DNA. The small piece of DNA migrates very quickly in an electrophoretic field, an electrical field through a gel, but if a protein sticks to it, that larger complex migrates more slowly and is retarded.

He was able to show that the PXR receptor can stick to this sequence in the promoter for CYP3A, but it does so, and retards it, only in concert with another member of the family. So the regulation of this is through a heterodymanic nuclear receptor process.