This technology includes monoclonal antibody (mAb) that binds to the junction region of the PAX3-FOXO1 and PAX7-FOXO1 fusion protein for the diagnosis of Alveolar Rhabdomyosarcoma (ARMS). Specifically, two monoclonal antibodies (PFM.1 and PFM.2) have been isolated that recognize the 92kDa bands found uniquely to the pediatric striated muscle tumors of the type Alveolar Rhabdomyosarcoma (ARMS) carrying the characteristic t(2;13)(q35;q14) or t(1;13)(p36;q14) chromosomal translocations. This in-frame fusion protein is unique to the most common type of ARMS and acts as a potent transcriptional activator as the result of a decreased regulatory (of PAX3/7) sensitivity of the C-terminal domain of FOXO1. The mAbs recognize the ARMS fusion protein PAX3/7 FKHR on western analysis and do not recognize PAX3/7 or FOXO1 alone. The antibodies work by Westerns, Co-Immunoprecipitation, Chromatin Immunoprecipitation Sequencing (CHipSeq), and immunohistochemistry.