Embryonal Rhabdomyosarcoma : MSA
ACTINS: Actin is found in virtually every cell type. Cells showing muscle differentiation have high concentration of actins and unique isoforms. Antibodies have been generated to specific isoforms (muscle vs. non-muscle actins) and to specific muscle types (skeletal vs. smooth muscle). Muscle-specific actin (MSA; HHF-35) (shown in this image) is a sensitive marker of rhabdomyosarcoma (RMS) but lacks specificity. It is also positive in leiomyosarcomas. It may be negative in poorly-differentiated RMS. Smooth muscle actin (SMA) is a sensitive marker of tumors with smooth muscle or myofibroblastic differentiation. However, it may be found in up to 13% of RMS.