CD99 and Ewing sarcoma: An attractive tumour target expressed on the surface of EWS cells is represented by CD99 antigen, which is a heavily O-glycosylated transmembrane protein with a molecular weight of 32 kDa that results to be expressed in Ewing sarcoma [12], synovial sarcoma (SS), and low-grade fibromyxoid sarcoma (LGFMS) [13]; the down-regulation of CD99 in EWS cell lines reduced their ability to form tumours and bone metastases in immunodeficient mice and decreased their tumourigenic and metastatic features in vitro [13].