Calponin is a polypeptide with a molecular weight of 34 kDa that interacts with actin, tropomyosin, and calmodulin. It is primarily found in smooth muscle tissue and is involved in the mechanisms of smooth muscle contraction. Calponin is a calcium-binding protein and its main function is to tonically inhibit the ATPase activity of myosin in smooth muscle. When calponin is phosphorylated by a protein kinase, which is dependent on calcium binding to calmodulin, the inhibition of smooth muscle ATPase by calponin is released.

The calponin antibody has proven to be useful in distinguishing benign sclerosing lesions of the breast from carcinoma. It has also been observed to be positive in malignant myoepithelioma and pleomorphic adenoma of salivary gland origin, as well as in angiomatoid malignant fibrous histiocytoma.