BCL9 and leukemia: Only one isoform of the MEF2D-HNRNPUL1 fusion was identified, involving exon 8 of MEF2D and exon 12 of HNRNPUL1. Expression of the fusion proteins in patient leukaemic cells was confirmed by immunoblotting (Supplementary Fig. 3) and the disruption of MEF2D and MEF2D-BCL9 rearrangements were shown to be present in the majority of leukaemic cells by fluorescence in situ hybridization (FISH), consistent with the fusions being leukaemia-initiating events acquired early in leukaemogenesis (Supplementary Fig. 4).