A group of highly cellular primitive round cell neoplasms which occur extracranially in soft tissue and bone and are derived from embryonal neural crest cells. These tumours occur primarily in children and adolescents and share a number of characteristics with Ewing's Sarcoma (Ewing's sarcoma). They may arise from the chest wall, skin, orbit, kidney, and other structures and tend to be locally invasive or metastasize, although relatively benign forms may occur. Characteristic histologic features include a tendency to form Homer-Wright rosettes and to stain positively with neuron-specific enolase and vimentin.