Caldesmon is a marker for smooth muscle differentiation. Found in smooth muscle and other tissues, caldesmon interacts with Ca2+-calmodulin, actin, tropomyosin, myosin, and phospholipids. It is more specific to smooth muscle differentiation than desmin and muscle specific actin. It inhibits the ATPase activity of myosin in smooth muscle, and mediates Ca2+-dependent inhibition of smooth muscle and non-muscle contraction. Caldesmon expression is found in Gastrointestinal Stromal Tumors (GIST), and can be used to differentiate epithelioid mesothelioma from serous papillary carcinoma of the ovary. It is also a specific marker for smooth muscle cells (SMC) and its neoplasms, and therefore can be used in the study of the SMC differentiation process as well as the differentiation of other tumors with SMC-like differentiation, including leiomyosarcoma and myofibroblastic tumors.