Myoglobin, an intracellular hemoprotein expressed in the heart and oxidative skeletal myofibres of vertebrates, binds molecular oxygen and may facilitate oxygen transport from erythrocytes to mitochondria, thereby maintaining cellular respiration during periods of high physiological demand. Anti-myoglobin labels skeletal and cardiac muscle cells. In combination with other striated muscle markers such as vimentin and myogenin, myoglobin is helpful in the identification of rhabdomyosarcoma and tumors with skeletal muscle differentiation. Recently, myoglobin has been reported to be expressed on epithelial cancer cells due to changed metabolic and environmental conditions. Myoglobin expression on cancer cells may play a causative role in tumor progression.