MyoD1 is a phosphorylated protein with a molecular weight of 45kDa encoded by the MyoD gene, which is expressed only in embryonic rhabdomyosarcoma cells, but not in normal adult rhabdomyosarcoma cells. MyoD1, one of the MyoD family of myogenic helix-loop-helix transcription factors, combined with myogenin, plays a role in coordinating the differentiation of skeletal muscle. MyoD1 is expressed in myoblasts before differentiation. Anti-MyoD1 immunostaining identifies cells committed to myogenesis in their earliest phase.2 In conjunction with other markers, anti-MyoD1 may be used for identification of rhabdomyosarcoma.^1,2