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 cells, may play a crucial role in retinoic acid-induced differentiation along the neural lineage, regulating gene induction and H3 'Lys-4' methylation at key developmental loci. May also play an indirect or direct role in endosomal transport (By similarity). 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 (By similarity). Interacts with ASH2L; the interaction is direct (By similarity). Interacts with ARFGEF1. 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). Belongs to the dpy-30 family.