CD99 and Ewing sarcoma: The tumor clusters expressed several genes that are known to be expressed by Ewing sarcoma tumor cells, including the diagnostic marker CD99 and EWS-FLI1 candidate target genes CAV1, CCND1, HES1, KDSR, and PAPPA (Fig. 1C and D; Supplementary Fig. S2F and S4A; refs. 21–23).