Transcriptional regulator (By similarity). Component or associated component of some histone methyltransferase complexes which regulates transcription through recruitment of those complexes to gene promoters (By similarity). Component of the Set1/Ash2 histone methyltransferase (HMT) complex, a complex that specifically methylates 'Lys-4' of histone H3, but not if the neighboring 'Lys-9' residue is already methylated (By similarity). As part of the MLL1/MLL complex it is involved in methylation and dimethylation at 'Lys-4' of histone H3 (By similarity). May play a role in hematopoiesis (By similarity). In association with RBBP5 and WDR5, stimulates the histone methyltransferase activities of KMT2A, KMT2B, KMT2C, KMT2D, SETD1A and SETD1B (By similarity). Interacts with HCFC1 (By similarity). Core component of several methyltransferase-containing complexes including MLL1/MLL, MLL2/3 (also named ASCOM complex) and MLL4/WBP7 (By similarity). 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 PAGR1, BAP18, CHD8, E2F6, HCFC1, HCFC2, HSP70, INO80C, KDM6A, KANSL1, LAS1L, MAX, MCRS1, MEN1, MGA, KAT8/MOF, NCOA6, PAXIP1/PTIP, PELP1, PHF20, PRP31, RING2, RUVB1/TIP49A, RUVB2/TIP49B, SENP3, TAF1, TAF4, TAF6, TAF7, TAF9, TEX10 and alpha- and beta-tubulin (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). Found in a complex with RBBP5, ASH2L, DPY30, KMT2A, KMT2D and WDR5 (PubMed:21335234). Component of a histone methylation complex composed of at least ZNF335, RBBP5, ASH2L and WDR5; the complex may have histone H3-specific methyltransferase activity, however does not have specificity for 'Lys-4' of histone H3 (By similarity). Within the complex, interacts with ZNF335 (By similarity). Interacts with RBBP5 (By similarity). Components of this complex may associate with components of a nuclear receptor-mediated transcription complex to form a complex at least composed of ZNF335, HCFC1, CCAR2, EMSY, MKI67, RBBP5, ASH2L and WDR5 (By similarity). Within this complex also interacts with CCAR2 and EMSY (By similarity). Interacts with DPY30 (By similarity). Interacts with SETD1A and SETD1B (By similarity). Ubiquitously expressed, with abundant expression in the heart, skeletal muscle and kidney. Low expression is seen in spleen, lung and testis. Both monomethylated and dimethylated on arginine residues in the C-terminus. Arg-291 is the major site. Methylation is not required for nuclear localization, nor for MLL complex integrity or maintenance of global histone H3K4me3 levels (By similarity).