Calponin is a smooth muscle specific, actin-, tropomyosin- and calmodulin-binding protein thought to be involved in regulation of actomyosin as well as the regulation or modulation of contraction. It is expressed on smooth muscle cells and myoepithelial cells. Calponin has been used to identify invasion of breast lesion. Additionally, Calponin is expressed on malignant fibrous histiocytoma of bone and adenoid cystic carcinoma of salivary gland. The consistently positive staining pattern in adenoid cystic carcinomas may be useful in discriminating histologically similar but consistently negative polymorphous low-grade adenocarcinomas