The identification of HRX and its partner genes is offering new insights into the genetic basis of the 11q23 leukaemias. Although some patterns and associations between the partner genes are beginning to emerge, it is not yet possible to frame a single unifying hypothesis for 11q23 leukaemic transformation. The study of transcriptional controls in other species, especially Drosophila and yeast, is offering possible clues as to the function of HRX and some of its partners. The identification of more partner genes may help to resolve these questions. The recognized poor prognosis of 11q23 leukaemias has prompted important variations in clinical trials. The molecular analysis of 11q23 events has therefore been particularly important in generating new molecular tools for the diagnosis and monitoring of disease in these patients. Ultimately, an understanding of 11q23 leukaemogenesis may open up new avenues for the molecular therapy of this disease.