Myeloproliferative disorders (MPD) and myelodysplastic syndromes (MDS) represent an overlapping spectrum of clonal preleukaemic conditions in which mutations have resulted in dysregulation of multipotent haemopoietic progenitors. An interstitial deletion of the long arm of chromosome 20 is a recurring abnormality associated with both of these classes of disorders. The association of 20q deletions with ‘stem cell’ disorders suggests that the deletion may mark the site of one or more genes, loss or inactivation of which perturbs the regulation of haemopoietic progenitors. The identification and study of the target gene(s) on chromosome 20q is therefore likely to illuminate the regulation of normal haemopoietic differentiation as well as the genesis of clonal myeloid disorders. This article provides a review of progress in the search for critical genes harboured within 20q deletions, together with a summary of several insights that these studies have provided into the biology of MPD and MDS.