In a study of Crohn’s Disease patients in China, serum levels of Vitamin D were not only lower than in healthy controls, but those levels inversely correlated with the Crohn’s Disease Activity Index in the patients, and directly correlated with levels of expression of TJ proteins (occludin, claudin-1, ZO-1 and JAM-A) in the GI mucosa [393]. Here, CLDN1 is linked to Crohn disease.