Essential component of a MLL/SET1 histone methyltransferase (HMT) complex, a complex that specifically methylates 'Lys-4' of histone H3 (H3K4). Functions as a transcriptional regulator. Binds to the TERT promoter and represses telomerase expression. Plays a role in TGFB1-mediated inhibition of cell-proliferation, possibly regulating SMAD3 transcriptional activity. Represses JUND-mediated transcriptional activation on AP1 sites, as well as that mediated by NFKB subunit RELA. Positively regulates HOXC8 and HOXC6 gene expression. May be involved in normal hematopoiesis through the activation of HOXA9 expression. Component of the MLL-HCF complex, at least composed of KMT2A/MLL1, MEN1, ASH2L, RBBP5, DPY30, WDR5, HCFC1 and HCFC2. Component of the menin-associated histone methyltransferase complex, at least composed of KMT2B/MLL4, MEN1, ASH2L, RBBP5, DPY30 and WDR5. Interacts with POLR2B. Interacts with POLR2A phosphorylated at 'Ser-5', but not with the unphosphorylated, nor 'Ser-2' phosphorylated POLR2A forms. Interacts with FANCD2 and DBF4. Interacts with JUND (via MBM motif); inhibits the interaction of JUND with MAPK10 and the phosphorylation of JUND by MAP kinases MAPK8 and MAPK10.