PAX3 and alveolar rhabdomyosarcoma: The two primary subtypes of RMS encountered in the pediatric population, ERMS and ARMS, can be distinguished molecularly as nearly 85% of ARMS are defined by gene fusions between PAX3-FOXO1 or PAX7-FOXO11, whereas ERMS are characterized by disparate mutations in the RAS pathway, effectors of the PI3 Kinase pathway, or in genes that control the cell cycle2.