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Soft tissue sarcomas (STSs) gather over 80 histological entities, with even more molecular subsets, characterised by a low to very low incidence in all populations. The majority of sarcomas arise from the soft tissue (close to 75%), with 15% gastrointestinal stromal tumours (GISTs) and 10% bone sarcomas. These ESMO-EURACAN (European Society for Medical Oncology-European Reference Network for rare adult solid cancers) Clinical Practice Guidelines cover STSs, while GISTs are covered by dedicated ESMO-EURACAN Clinical Practice Guidelines [1]. Kaposi's sarcoma is not considered in the present document. Extraskeletal Ewing and Ewing-like sarcoma is covered by ESMO Clinical Practice Guidelines on bone sarcomas [2]. In general, the

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HeLa cells were encapsulated in agarose microbeads, permeabilized and incubated with Br‐UTP in a ‘physiological’ buffer; then sites of RNA synthesis were immunolabelled using an antibody that reacts with Br‐RNA. After extending nascent RNA chains by < 400 nucleotides in vitro, approximately 300–500 focal synthetic sites can be seen in each nucleus by fluorescence microscopy. Most foci also contain a component of the splicing apparatus detected by an anti‐Sm antibody. alpha‐amanitin, an inhibitor of RNA polymerase II, prevents incorporation into these foci; then, using a slightly higher salt concentration, approximately 25 nucleolar foci became clearly visible. Both nucleolar and extra‐nucleolar foci remain after nucleolytic removal of approximately 90% chromatin. An underlying structure probably organizes groups of transcription units into ‘factories’ where transcripts are both synthesized and processed.

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