A protein your body makes that opens the gaps between intestinal cells—too much zonulin may contribute to ‘leaky gut’ conditions.Zonulin (pre-haptoglobin 2) is an endogenous regulator of intestinal tight junction permeability. It binds to EGF receptor and PAR2 on enterocytes, triggering cytoskeletal rearrangement that opens tight junctions and increases paracellular permeability. Elevated zonulin levels are associated with celiac disease, type 1 diabetes, and other autoimmune conditions. Larazotide (AT-1001) is a synthetic octapeptide designed to antagonize zonulin-mediated tight junction opening, representing the only therapeutic specifically targeting this permeability pathway.