ACTA2 / Smooth Muscle Actin (SMA) belongs to the actin family of proteins, which are highly conserved proteins that play a role in cell motility, structure and integrity. In immunohistochemistry, ACTA2 is to define smooth muscle cells and myofibroblasts in normal and malignant tissues. It is a common myoepithelial marker for normal and cancerous breast, salivary and sweat gland tissue. High levels of ACTA2 expression are correlated with aggressiveness, invasion and poorer prognosis in breast cancers positive for EGFR and ERBB2, basal cell carcinoma, and pancreatic ductal adenocarcinoma. ACTA2 can also be used to differentiate pleuropulmonary desmoid tumors (positive) from solitary fibrous tumors (negative). ACTA2 has membranous and cytoplasmic staining, and in normal tissue it is positive in smooth muscle cells, breast myoepithelial cells, chrondrocytes, decidual stromal cells, myofibroblasts, osteoblasts, pericytes, and salivary and sweat gland cells. References: Pancreas. 2010 Nov;39(8):1254-1262, PMID: 20467342; Oncotarget 2016 Jul; 8.31; 50570-50581, PMID: 28881584; Hum Pathol. 2010 Aug;41(8):1128-37, PMID: 20381122; Arch Pathol Lab Med 2006;130:1503, PMID: 17090192