EWSR1 and mesenchymal cell neoplasm: Ewing sarcoma (EWS), a high-grade mesenchymal neoplasm composed of sheets of small round blue cells with strong CD99 membrane expression, is characterized by a specific reciprocal chromosomal translocation merging a member of the FET family of proteins with several members of the ETS family of transcription factors (in 85–90% of cases, the chromosomal translocation t (11; 22) (q24; q12) is observed, generating the fusion product EWSR1-FLI1) [67,68,69].