Retatrutide has already generated enormous interest because of its clinical development program and its potential role as a next-generation obesity and metabolic therapy. But there is another battle surrounding retatrutide that has nothing to do with its weight-loss results. It is a legal and regulatory battle over something much more fundamental: What exactly is retatrutide under U.S. federal law? That question is now heading directly into the U.S. Court of Appeals for the Seventh Circuit. On September 21, 2026, the U.S. Department of Justice filed a letter in the ongoing Eli Lilly and Company v. FDA litigation, directing the Seventh Circuit's attention to a very recent decision involving an unexpected name in this story: Chobani, the yogurt company. The case, Franco v. Chobani, involves FDA's interpretation of a scientific regulatory definition. The DOJ is pointing to that decision as relevant to the question of how much weight a court should give the FDA when the agency is interpreting technical language contained in its own regulations. And with oral arguments scheduled for Thursday, September 24, the timing is significant. The basic dispute At the center of the case is the FDA's regulatory definition of a "protein." Under the relevant regulation, a protein is defined as an alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size. The FDA determined that retatrutide does not meet that definition. The agency's reasoning is based on the structure of the molecule. FDA concluded that retatrutide contains 40 alpha amino acids and one non-alpha amino acid, meaning it does not satisfy the regulatory requirement for a protein. Lilly challenged that determination. The resulting litigation is therefore not simply about whether retatrutide is scientifically a peptide or protein in the ordinary sense. It is about how the federal regulatory definition should be interpreted and who gets to make that determination.