Calponin, a calmodulin, is a thin filament associated protein that is implicated in the regulation and modulation of smooth muscle contraction. It is capable of binding to actin, calmodulin, troponin C and tropomyosin. Calponin is expressed in smooth muscle and tissues containing significant amounts of 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.