jats:titleAbstract</jats:title>jats:secjats:titleBackground</jats:title>jats:pImproving the poor prognosis of infant leukaemias remains an unmet clinical need. This disease is a prototypical fusion oncoprotein‐driven paediatric cancer, with jats:italicMLL</jats:italic> (jats:italicKMT2A</jats:italic>)‐fusions present in most cases. Direct targeting of these driving oncoproteins represents a unique therapeutic opportunity. This rationale led us to initiate a drug screening with the aim of discovering drugs that can block MLL‐fusion oncoproteins.</jats:p></jats:sec>jats:secjats:titleMethods</jats:title>jats:pA screen for inhibition of MLL‐fusion proteins was developed that overcomes the traditional limitations of targeting transcription factors. This luciferase reporter‐based screen, together with a secondary western blot screen, was used to prioritize compounds. We characterized the lead compound, disulfiram (DSF), based on its efficient ablation of MLL‐fusion proteins. The consequences of drug‐induced MLL‐fusion inhibition were confirmed by cell proliferation, colony formation, apoptosis assays, RT‐qPCR, in vivo assays, RNA‐seq and ChIP‐qPCR and ChIP‐seq analysis. All statistical tests were two‐sided.</jats:p></jats:sec>jats:secjats:titleResults</jats:title>jats:pDrug‐induced inhibition of MLL‐fusion proteins by DSF resulted in a specific block of colony formation in jats:italicMLL</jats:italic>‐rearranged cells in vitro, induced differentiation and impeded leukaemia progression in vivo. Mechanistically, DSF abrogates MLL‐fusion protein binding to DNA, resulting in epigenetic changes and down‐regulation of leukaemic programmes setup by the MLL‐fusion protein.</jats:p></jats:sec>jats:secjats:titleConclusion</jats:title>jats:pDSF can directly inhibit MLL‐fusion proteins and demonstrate antitumour activity both in vitro and in vivo, providing, to our knowledge, the first evidence for a therapy that directly targets the initiating oncogenic MLL‐fusion protein.</jats:p></jats:sec>