Cystatin C is a 14 kDa member of the Cystatin superfamily of cysteine protease inhibitors (1). Most cell types secrete Cystatin C (1). Cystatin C inhibits cathepsins, and thereby may function as a tumor suppressor by inhibiting cathepsin mediated tumor cell invasion (2). In addition, this tumor suppressor function can also be attributed to Cystatin C's ability to antagonize TGF-β1 signaling (2,3). Cystatin C may also modulate antigen presentation through its ability to inhibit cathepsins (4). Cystatin C is also a biomarker for kidney dysfunction (5).