There is аn аctivаtоr оn the enhancer, and general transcriptiоn factors complementary to the activator. The DNA segment is in a u-turn shape after the enhancer, and RNA polymerase two is attached to the transcription factors and the DNA directly upstream of gene x, which makes the complex of activator, general transcription factors and RNA polymerase two span from the enhancer to directly before gene X. There is an arrow below gene X pointing away from the complex labeled expression of gene x. The figure above depicts the DNA-protein complex that is assembled at the transcriptional start site of gene X when the expression of gene X is activated in liver cells. Previous studies have shown that gene X is never expressed in nerve cells. Based on the diagram, which of the following most likely contributes to the specific expression pattern of gene X ?