Recently, CTCs have been isolated from 18 patients diagnosed with Ewing sarcoma, based on the immunomagnetic separation of CD99-positive tumor cells [241]; EWSR1/FLI1 or EWSR1/ETS-related gene transcripts have been confirmed through quantitative and digital RT-PCR on cDNA obtained from CTCs, with a limit of detection of 1 cell/mL of PB. This evidence concerns the gene CD99 and Ewing sarcoma.