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. This gene encodes a cystatin from the type 2 family, which is down-regulated in metastatic breast tumor cells as compared to primary tumor cells. Loss of expression is likely associated with the progression of a primary tumor to a metastatic phenotype.[7]
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
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Zeeuwen PL, Van Vlijmen-Willems IM, Jansen BJ, et al. (2001). "Cystatin M/E expression is restricted to differentiated epidermal keratinocytes and sweat glands: a new skin-specific proteinase inhibitor that is a target for cross-linking by transglutaminase". J. Invest. Dermatol. 116 (5): 693–701. doi:10.1046/j.1523-1747.2001.01309.x. PMID11348457.
Zeeuwen PL, Dale BA, de Jongh GJ, et al. (2003). "The human cystatin M/E gene (CST6): exclusion candidate gene for harlequin ichthyosis". J. Invest. Dermatol. 121 (1): 65–8. doi:10.1046/j.1523-1747.2003.12312.x. PMID12839564.
Song J, Jie C, Polk P, et al. (2006). "The candidate tumor suppressor CST6 alters the gene expression profile of human breast carcinoma cells: down-regulation of the potent mitogenic, motogenic, and angiogenic factor autotaxin". Biochem. Biophys. Res. Commun. 340 (1): 175–82. doi:10.1016/j.bbrc.2005.11.171. PMID16356477.