Benign Skeletal Muscle Soft Tissue Tumors. Myoglobin immunohistochemical stain of rhabdomyoma, a benign tumor of skeletal muscle. This demonstrates the presence of differentiated myogenic cells, or rhabdomyoblasts, with abundant cytoplasmic myoglobin, indicated by the dark brown staining. Immunohistochemical Markers of Soft Tissue Tumors: Pathologic Diagnosis, Genetic Contributions, and Therapeutic Options. Parham DM - Analytical chemistry insights (2015). Not Altered. CC.