Disclosed are Cystatin C (CysC) homologues, including CystC homologues that act as antagonists or inhibitors of transforming growth factor-β (TGF-β). Also disclosed are methods to identify CystC homologues that are antagonists or inhibitors of TGF-β and compositions and therapeutic methods using CystC and homologues thereof to regulate the activity of TGF-β, and TGF--βmediated tumor malignancy and invasion and other TGF--βmediated fibrotic or proliferative conditions and diseases.