MyoD1 is a protein with a key role in regulating muscle differentiation. It regulates muscle cell differentiation by inducing cell cycle arrest, a prerequisite for myogenic initiation. The protein is also involved in muscle regeneration. MyoD1 is expressed in developing skeletal muscle tissue but faintly in adult skeletal muscle. In abnormal tissues, it labels tumor cells in rhabdyosarcoma. MyoD1 is one of the earliest markers of myogenic commitment. Anti-MyoD1 has been useful in differentiating rhabdomyosarcomas from other tumors. It is a sensitive and specific marker for myogenic differentiation.