Cystatin-SA, also known as Cystatin-2, Cystatin-S5 and CST2, is a secreted protein which belongs to the cystatin family. Cystatin-2 / CST2 is expressed in submandibular and sublingual saliva but not in parotid saliva (at protein level). It is also expressed in submandibular gland and parotid gland. The cystatin superfamily encompasses proteins that contain multiple cystatin-like sequences. Some of the members are active cysteine protease inhibitors, while others have lost or perhaps never acquired this inhibitory activity. There are three inhibitory families in the superfamily, including the type 1 cystatins (stefins), type 2 cystatins and the kininogens. The type 2 cystatin proteins are a class of cysteine proteinase inhibitors found in a variety of human fluids and secretions, where they appear to provide protective functions. The CST1, CST2, CST4, and CST5 are expressed in differential, tissue-specific patterns. Expression of CST2 and CST5 is restricted to the submandibular and parotid glands, while CST1 and CST4 are expressed in these tissues and in the lacrimal gland.