PAX3 and embryonal rhabdomyosarcoma: The first molecular tools for substratification of RMS came with the definition of the fusion of FOXO1 with PAX3 or PAX7, which splits tumors into two major subtypes and roughly correlates with the histologic features: ERMS and alveolar RMS.15,16 More recently, data suggest that some of the previously categorized ERMS tumors have a distinctive morphology composed of elongated spindle cells and often sclerotic stroma.