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Haematopoiesis — the process by which blood cells, including all cells of the immune system, develop — is of obvious importance for the function of the immune system. Many studies have been carried out to investigate how haematopoietic cells are generated from multipotent stem cells during embryonic development, as reviewed by Isabelle Godin and Ana Cumano on page 593 of this issue. Two pools of haematopoietic precursors that have different characteristics have been identified in the yolk sac and the para-aortic splanchnopleura. This article outlines the story of the haematopoietic race between these two pools of cells — the 'hare' and the 'tortoise', respectively.

Defects in haematopoiesis can result in life-threatening complications, as seen in patients who have X-linked severe combined immunodeficiency (SCID). However, as discussed on page 615, in an Opinion article by Alain Fischer, Salima Hacein-Bey and Marina Cavazzana-Calvo, developments in gene-transfer technologies have enabled gene therapy for these immunodeficiencies to become an exciting reality. In a highlight on page 454, recent progress in gene therapy for another form of SCID, which occurs in patients who lack adenosine deaminase (ADA-SCID), is described.

Two other reviews in this issue also discuss new approaches for immunotherapy. Clotilde Théry, Laurence Zitvogel and Sebastian Amigorena (page 569) outline the composition, biogenesis and functions of exosomes (small membrane vesicles), and their possible applications for the treatment of cancer. On page 557, Mitchell Kronenberg and Laurent Gapin describe Vα14i natural killer T cells, in terms of their development, selection, function and possible role in future immunotherapies.

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Bell, E., Bell, J. & Buckland, J. In this issue. Nat Rev Immunol 2, 537 (2002).

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