The smooth muscle actin (SMA) marker has been widely used in various contexts. The two commonly used antibodies for SMA detection are anti-SMA and muscle-specific actin (clone HHF-35). SMA is known as a sensitive marker for myoepithelial differentiation. However, it lacks specificity as any cell with significant actin expression will show positive staining for SMA. In breast tissue, myofibroblasts and blood vessels typically stain positive for SMA. This can pose challenges in cases where there are myofibroblasts or blood vessels in close proximity to the epithelial lesion being examined, such as in sclerosing adenosis, radial scars, papillary lesions, or ductal carcinoma in situ (DCIS).

Despite its lack of specificity, SMA remains a useful marker due to its high sensitivity. It helps in resolving differential diagnoses in a significant number of cases. By considering the staining pattern and context, pathologists can interpret the results and differentiate between different lesions.