Sarcomas comprise some of the most aggressive solid tumours that, for the most part, respond poorly to chemo- and radiation therapy and are associated with a sombre prognosis when surgical removal cannot be performed or is incomplete. Partly because of their lower frequency, sarcomas have not been studied as intensively as carcinomas and haematopoietic malignancies, and the molecular mechanisms that underlie their pathogenesis are only beginning to be understood. Even more enigmatic is the identity of the primary cells from which these tumours originate. Over the past 25 years, however, several non-random chromosomal translocations have been found to be associated with defined sarcomas. Each of these translocations generates a fusion gene believed to be directly related to the pathogenesis of the sarcoma in which it is expressed. The corresponding fusion proteins provide a unique tool not only to study the process of sarcoma development, but also to identify cells that are permissive for their putative oncogenic properties. This is the first of two reviews that cover the mechanisms whereby specific fusion/mutant gene products participate in sarcoma development and the cellular context that may provide the necessary permissiveness for their expression and oncogenicity. Part 1 of the review focuses on sarcomas that express fusion genes containing TET gene family products, including EWSR1, TLS/FUS, and TAFII68. Part 2 (J Pathol 2007; DOI: 10.1002/path.2008) summarizes our current understanding of the genetic and cellular origins of sarcomas expressing fusion genes exclusive of TET family members; it also covers soft tissue malignancies harbouring specific mutations in RTK-encoding genes, the prototype of which are gastrointestinal stromal tumours (GIST). Copyright (c) 2007 Pathological Society of Great Britain and Ireland.