Mouse RMS cell lines were derived from primary or metastatic tumors in transgenic mice designed to develop ERMS or ARMS: cell lines U23674 and U48484 represent the ARMS subtype as they are derived from mice with forced expression of the Pax3-FOXO1 fusion protein (Keller and Capecchi, 2005), while cell lines U33915 and U57810 represent the ERMS subtype, which does not express the fusion protein but instead is driven by mutations to PTCH1 and p53 in a muscle-specific fashion (Rubin et al., 2011). The gene discussed is PAX3; the disease is embryonal rhabdomyosarcoma.