BCL9 and acute lymphoblastic leukemia: However, the MEF2D-BCL9 rearrangements involve only the last one or two of ten exons of BCL9 exons resulting in loss of the BCL9 domains that mediate interaction with PYGO1 and β-catenin, consistent with the lack of enrichment of WNT/β-catenin signalling in the gene expression profile of MEF2D-rearranged ALL.