This post investigates a claim in a review by Morrel and coworkers that excessive dietary fructose can decrease the expression of Ctr1 in the duodenum. [1] How could this happen? What is transcription factor for Ctr1? As it turns out, both Cu+ and Cu2+ can bind to the CTR1 transcription factor Sp1 and negatively regulate its activity [2,3] The featured image describes out Cu can disrupt the Zn2+ finger(s?) Sp1 uses to bind to the promoters of itself and CTR1. If you wish to skip the details of some brilliant experiments, click to go to the lay friendly summary.