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The histopathologic diagnosis of Ewing's sarcoma has always been difficult, and its histogenesis has never been elucidated. Developments in the molecular pathology of the sarcoma and cytogenetically related tumors have not only provided considerable diagnostic assistance but also have opened new views on the classification of a formerly heterogeneous group of neoplasms. Chromosome translocations t(11;22) (q24;q12) and t(21;22) (q22;1q12) and their chimeric fusion oncoproteins are now the hallmarks of the Ewing's sarcoma/peripheral primitive neuroectodermal tumor family. In this review, we present the molecular basis of this family and its interrelationships, summarize advances in diagnosis, and relate these to prognosis. Finally, histogenetic (developmental, embryologic, and microenvironmental) theories are discussed with the conclusion that the histogenesis of this subset of sarcomas does not necessarily follow a genetic commonality.