Recognizes theα-smooth muscle isoform of actin. It shows no cross reaction with actin from fibroblasts (βand γ-cytoplasmic), striated muscle (α-sarcomeric), and myocardium (α-myocardial). Its epitope is composed of the acetyl group and the first 4 amino acids on the N-terminal end of the peptidic chain of a-smooth actin. This 1A4 Mab stains smooth muscle cells in vessel walls, gut wall, and myometrium. Myoepithelial cells in breast and salivary gland are also stained as they also contain this actin. This MAb is reportedly useful for identifying tumors arising from smooth muscle and myoepithelial cells.