CD99 and Ewing sarcoma: Skeletal and extraskeletal Ewing's sarcoma (ES) and peripheral primitive neuroectodermal tumours (pPNET) are characterised by a high expression of the CD99 antigen (Ambros et al, 1991) and the presence of the balanced translocation t(11;22)(q24;q12) or related aberrations involving 22q12 (Aurias et al, 1983; Turc-Carel et al, 1983; Becroft et al, 1984; Whang-Peng et al, 1984).