A 34-kD regulatory protein linked to actin filaments,calponin interacts with calmodulin, tropomyosin, andactin. It is unique to smooth muscle tissue andmyoepithelial cells and is involved in the mechanism ofsmooth muscle contraction. It has been discovered thatanti-calponin is a helpful marker for distinguishingbetween invasive cancer and benign sclerosing breasttumours.