RAG2 and neoplasm: In this study, we induced zRMS by overexpressing the rag2:kRASG12D human oncogene and utilized lineage tracing of the zRMS cell of origin to quantify tumour progression in vivo in genetically modified zebrafish, where we identified new roles for the zebrafish transcription factor Pax3a and its paralogue Pax3b in fusion negative RMS.