Moreover, the detection of CD99 is routinely used for the differential diagnosis of conventional Ewing sarcoma with respect to other types of small round-cell tumors of childhood, including lymphomas, small round-cell osteosarcoma, and the most recently defined Ewing-like round cell tumors, a group of very rare malignancies morphologically resembling Ewing sarcoma but carrying a different type of genetic alterations and exhibiting lower and dispersed expression of CD99 [1,2,61,62,63]. This evidence concerns the gene CD99 and Ewing sarcoma.