Histone methyltransferase that catalyzes methyl group transfer from S-adenosyl-L-methionine to the epsilon-amino group of 'Lys-4' of histone H3 (H3K4) (PubMed:25561738). Part of chromatin remodeling machinery predominantly forms H3K4me1 methylation marks at active chromatin sites where transcription and DNA repair take place (PubMed:25561738, PubMed:17500065). Acts as a coactivator for estrogen receptor by being recruited by ESR1, thereby activating transcription (PubMed:16603732). L-lysyl(4)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-homocysteine Component of the MLL2 complex (also named ASCOM complex), at least composed of catalytic subunit KMT2D/MLL2, ASH2L, RBBP5, WDR5, NCOA6, DPY30, KDM6A, PAXIP1/PTIP, PAGR1 and alpha- and beta-tubulin (PubMed:23508102, PubMed:17500065, PubMed:12482968, PubMed:16603732, PubMed:17021013, PubMed:17761849, PubMed:17851529). Forms a core complex with the evolutionary conserved subcomplex WRAD composed of WDR5, RBBP5, ASH2L/ASH2 and DPY30 subunits; WRAD differentially stimulates the methyltransferase activity (PubMed:25561738). Interacts with ESR1; interaction is direct (PubMed:16603732). Interacts (via WIN motif) with WDR5 (PubMed:22665483, PubMed:22266653). Expressed in most adult tissues, including a variety of hematoipoietic cells, with the exception of the liver. LXXLL motifs 5 and 6 are essential for the association with ESR1 nuclear receptor. The disease is caused by variants affecting the gene represented in this entry. This gene mapped to a chromosomal region involved in duplications and translocations associated with cancer. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily. Another protein KMT2B/MLL4, located on chromosome 19, was first named MLL2 (see AC Q9UMN6). Thus, KMT2B/MLL4 is often referred to as MLL2 and vice versa in the literature.