Calponin is a protein of 34-kD that binds to actin, tropomyosin, and calmodulin. It plays a role in the process of smooth muscle contraction and is found only in smooth muscle tissue. The use of anti-calponin has proven helpful in staining myoepithelium, making it useful for confirming the presence of myoepithelial cells in non-cancerous breast growths and their absence in invasive breast cancers. Calponin‘s presence has also been observed in myoepithelial tumors and pleomorphic adenomas of the salivary gland.