Source of progenitor cells, to repair in the event of MI. Activated in response to muscle damage, then expresses Myf-5 or MyoD, myogenin, and MRF4. Ability to expand in vitro, resist ischemia, and have myogenic differentiation. Surrounds the sarcolemma. Advantages include a reduced likelihood of an immune response reaction, resistance to hypoxic conditions, production of angionenic factors, and a contractle phenotype. On the other hand, arrhythmias could occur, whilst having a low survival rate and a high chance of rejection. This evidence concerns the gene MYF5 and cardiac arrhythmia.