Alternative mechanisms for oncogenic activation of MLL in hematologic malignancies. (A) Direct fusion of MLL with transcriptional effector domains results in the recruitment of transcription activation complexes to MLL target genes through the fusion partner moiety. (B) Fusion partner–induced dimerization/oligomerization of MLL results in the aberrant maintenance of target gene expression, possibly by the recruitment of cofactors or basal transcriptional machinery through the MLL moiety of the chimera. TAD indicates transactivation domain; DD, dimerization domain.