BCL9 and acute lymphoblastic leukemia: It remains to be established whether mutant MEF2B can drive HDAC9 expression in B-cell lymphomas, but recent research has identified a novel MEF2D–BCL9 fusion protein associated with high-risk acute B-cell precursor lymphoblastic leukemia (ALL) that directly upregulates HDAC9 (Suzuki et al., 2016).