Testatin is a member of the Cystatin family. Cystatins comprise genes that all show expression patterns that are strikingly restricted to reproductive tissue. Cystatins are a family of cysteine protease inhibitors with homology to chicken cystatin. There are typically about 115 amino acids in this family. They are largely acidic, contain four conserved cysteine residues known to form two disulfide bonds, may be glycosylated and/or phosphorylated, with similarity to fetuins, kininogens, stefins, histidine-rich glycoproteins and cystatin-related proteins. Testatin shows homology to family 2 cystatins, a group of broadly expressed small secretory proteins that are inhibitors of cysteine proteases in vitro but whose in vivo functions are unclear. It is expressed in germ cells and somatic cells in reproductive tissues. Testatin is considered a strong candidate for involvement in early testis development. Testatin expression is maintained in the adult Sertoli cell, and it can also be found in a small population of germ cells.