CD99 and Ewing sarcoma: Therefore, small-blue-round-cell tumours of childhood and early adolescence that show high CD99 expression and the presence of a 22q12 aberration are often grouped under the term ‘Ewing tumours’ (ETs) (Ambros et al, 1991) or ‘Ewing family of tumours’ (EFTs) (Delattre et al, 1994).