血清胱抑素C(cystatin C、cystatin 3、gamma trace、胱抑素C、半胱氨酸蛋白酶抑制剂、胱蛋白、后γ-球蛋白，或神经内分泌基本多肽)，是由CST3基因编码的一种蛋白质，其分子量小(13000)由体内有核细胞恒定产生，能自由通过(滤过)肾小球，且肾小管上皮细胞不分泌亦不重吸收，主要用作肾功能的生物标记。最近，已经研究了其在预测新发或恶化的心血管疾病的作用。它也似乎涉及类淀粉蛋白(一种特定类型的蛋白沉积)的脑功能障碍，如阿兹海默病。在人类中，所有细胞与细胞核(含DNA的细胞芯)产生具有120个氨基酸肽之血清胱抑素C。它几乎存在于所有的组织及体液中。它是一种溶酶体蛋白酶的酶抑制剂(来自于分解蛋白质的特定细胞亚单位的一种酶)，及可能是一种最重要的半胱氨酸蛋白酶(Cysteine protease)细胞外的(Extracellular)的抑制剂(经由的特定类型蛋白质降解酶、它可以防止细胞外蛋白质分解)。血清胱抑素C属于基因家族2型胱抑素(Cystatin)。

Cystatin C or cystatin 3 (formerly gamma trace, post-gamma-globulin or neuroendocrine basic polypeptide), a protein encoded by the CST3 gene, is mainly used as a biomarker of kidney function. Recently, it has been studied for its role in predicting new-onset or deteriorating cardiovascular disease. It also seems to play a role in brain disorders involving amyloid (a specific type of protein deposition), such as Alzheimer's disease. In humans, all cells with a nucleus (cell core containing the DNA) produce cystatin C as a chain of 120 amino acids. It is found in virtually all tissues and body fluids. It is a potent inhibitor of lysosomal proteinases (enzymes from a special subunit of the cell that break down proteins) and probably one of the most important extracellular inhibitors of cysteine proteases (it prevents the breakdown of proteins outside the cell by a specific type of protein degrading enzymes). Cystatin C belongs to the type 2 cystatin gene family.