Calponin, a calmodulin, binds tropomyosin and F-actin and is thought to be involved in the regulation of smooth muscle contraction. The expression of calponin is restricted to smooth muscle cells and is a marker of the differentiated contractile phenotype of developing smooth muscle. Two isoforms of calponin exist whose molecular weights are 34kDa and 29kDa. Expression of the 29kDa form is primarily restricted to muscle of the urogenital tract. The expression of calponin has also been demonstrated in myoepithelial cells from benign and malignant breast lesions.