{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"Calponin is a 34 kDa polypeptide that interacts with actin, tropomyosin, and calmodulin. It is involved in smooth-muscle contraction mechanisms and is restricted exclusively to smooth-muscle tissue. Calponin is a calcium-binding protein. Calponin tonically inhibits the ATPase activity of myosin in smooth muscle. Phosphorylation of calponin by a protein kinase (which is dependent upon calcium binding to calmodulin) releases the calponin’s inhibition of the smooth-muscle ATPase."}]},{"type":"paragraph","children":[{"type":"text","value":"Calponin has been found to be useful in differentiating benign sclerosing lesions of the breast from Carcinoma. Calponin positivity has also been noted in Malignant Myoepithelioma and Pleomorphic Adenoma of Salivary Gland origin, as well as in Angiomatoid Malignant Fibrous Histiocytoma."}]}]}