The cystatin superfamily is a well-established family of cysteine protease inhibitors. All true cystatins inhibit cysteine peptidases of the papain family, such as cathepsins, while some also inhibit legumain family enzymes. The CRES (cystatin-related epididymal spermatogenic) protein defines a new subgroup in the family 2 cystatins of the cystatin superfamily. CRES proteins lack two of the three consensus sites necessary for the cystatin inhibition of C1 cysteine proteases. They are also preferentially expressed in postmeiotic germ cells, the proximal caput epididymidis, and anterior pituitary gonadotrophs. Therefore, CRES proteins may perform unique and tissue-specific functions in the reproductive and neuroendocrine systems. As a member of the CRES subfamily, Cystatin-like 1 (CSTL1) is a 145 amino acid protein and is expressed in testis.

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