As part of the MLL1/MLL complex, involved in the methylation of histone H3 at 'Lys-4', particularly trimethylation. Histone H3 'Lys-4' methylation represents a specific tag for epigenetic transcriptional activation. May play some role in histone H3 acetylation. In embryonic stem (ES) cells, plays a crucial role in the differentiation potential, particularly along the neural lineage, regulating gene induction and histone H3 'Lys-4' methylation at key developmental loci, including that mediated by retinoic acid. Does not affect ES cell self-renewal. May also play an indirect or direct role in endosomal transport. Homodimer. Core component of several methyltransferase-containing complexes including MLL1/MLL, MLL2/3 (also named ASCOM complex) and MLL4/WBP7. Each complex is at least composed of ASH2L, RBBP5, WDR5, DPY30, one or more specific histone methyltransferases (KMT2A/MLL1, KMT2D/MLL2, KMT2C/MLL3 and KMT2B/MLL4), and the facultative components MEN1, HCFC1, HCFC2, NCOA6, KDM6A, PAXIP1/PTIP, PAGR1 and alpha- and beta-tubulin PAXIP1/PTIP, PAGR1 and alpha- and beta-tubulin. Interacts with ASH2L. The interaction with ASH2L is direct (By similarity). Interacts with ARFGEF1 (By similarity). Component of the SET1 complex, at least composed of the catalytic subunit (SETD1A or SETD1B), WDR5, WDR82, RBBP5, ASH2L/ASH2, CXXC1/CFP1, HCFC1 and DPY30 (By similarity). Associated with chromatin at regions enriched in histone H3 trimethylated at 'Lys-4'. Highly enriched in gene promoter regions and 5' UTRs, but not in downstream regions of genes or 3' UTRs. Belongs to the dpy-30 family.