In embryonic stem (ES) cells, plays a crucial role in the differentiation potential, particularly along the neural lineage, regulating gene induction and H3 'Lys-4' methylation at key developmental loci, including that mediated by retinoic acid (PubMed:21335234). Does not affect ES cell self-renewal (PubMed:21335234). Component or associated component of some histone methyltransferase complexes which regulates transcription through recruitment of those complexes to gene promoters (By similarity). As part of the MLL1/MLL complex, involved in mono-, di- and trimethylation at 'Lys-4' of histone H3 (By similarity). Histone H3 'Lys-4' methylation represents a specific tag for epigenetic transcriptional activation (By similarity). In association with ASH2L and WDR5, stimulates the histone methyltransferase activities of KMT2A, KMT2B, KMT2C, KMT2D, SETD1A and SETD1B (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). 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, MYST1/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 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). Interacts with ZNF335 (PubMed:23178126). Interacts with ASH2L; the interaction is direct (By similarity). Interacts with WDR5; the interaction is direct (By similarity). Components of the ZNF335-RBBP5-ASH2L-WDR5 histone methylation 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 interacts with EMSY (By similarity). Found in a complex with RBBP5, ASH2L, DPY30, KMT2A, KMT2D and WDR5 (PubMed:21335234). Interacts with SETD1A (By similarity). Interacts with WDR82 (By similarity).