Evaluation of new monoclonal anti-MyoD1 and anti-myogenin antibodies for the diagnosis of rhabdomyosarcoma

New monoclonal anti-MyoD1 and anti-myogenin antibodies were evaluated immun ohistochemically to determine whether they are useful in discriminating rha bdomyosarcoma (RMS) from other soft tissue tumors in routinely processed se ctions. Neither MyoD1 nor myogenin was expressed in normal, mature striated muscle. In RMS, nuclear expression of MyoD1 and myogenin was found in 82 a nd 80% of non-overlapping cases, respectively. MyoD1 was generally expresse d in small, primitive tumor cells, and larger cells exhibiting morphologica l evidence of skeletal muscle differentiation failed to express positive nu clear immunostaining. Positive nuclear staining for myogenin was stronger t han that for MyoD1 in cases with abundant differentiated tumor cells, but w as less prominent in cases in which small, primitive tumor cells predominat ed. No leiomyosarcomas, Ewing's sarcomas/peripheral primitive neuroectoderm al tumors or other soft tissue tumors exhibited nuclear expression of MyoD1 or myogenin. In conclusion, both anti-MyoD1 and anti-myogenin antibodies a re useful for diagnosing RMS and for discriminating RMS from other soft tis sue tumors.