Contributes to histone modification. May position the N-terminus of histone H3 for efficient trimethylation at 'Lys-4' . As part of the MLL1/MLL complex it is involved in methylation and dimethylation at 'Lys-4' of histone H3 . H3 'Lys-4' methylation represents a specific tag for epigenetic transcriptional activation. As part of the NSL complex it may be involved in acetylation of nucleosomal histone H4 on several lysine residues. May regulate osteoblasts differentiation. In association with RBBP5 and ASH2L, stimulates the histone methyltransferase activities of KMT2A, KMT2B, KMT2C, KMT2D, SETD1A and SETD1B.