Citrulline-specific probe-rhodamine hydrate is a specific probe for citrulline (Citrulline) combined with a rhodamine fluorescent group. Citrulline is the hydrolysis product of arginine catalyzed by protein arginine deiminase (PAD). PAD is abnormally activated in many diseases, leading to increased citrulline levels. Citrulline-specific probe-rhodamine hydrate is a biological probe that can identify diseases showing abnormal increases in PAD activity and may be effectively used in animal models of ulcerative colitis^[1].