EWSR1 and Ewing sarcoma: The pathognomonic feature of ES is the gene fusion of a member of the FET gene family (consisting of FUS, EWSR1 and TAF15), by far most frequently EWSR1 (EWS RNA binding protein 1), with a member of the ETS gene family of transcription factors, most commonly FLI1. The resulting fusion protein is causative for neoplastic transformation and tumour progression; other mutations at diagnosis are rare (Grünewald et al. 2018).