Histone methyltransferase that catalyzes methyl group transfer from S-adenosyl-L-methionine to the epsilon-amino group of 'Lys-4' of histone H3 (H3K4) via a non-processive mechanism (PubMed:25561738, PubMed:17355966). Part of chromatin remodeling machinery, forms H3K4me1, H3K4me2 and H3K4me3 methylation marks at active chromatin sites where transcription and DNA repair take place (PubMed:25561738, PubMed:17355966). Plays an essential role in regulating the transcriptional programming of multipotent hematopoietic progenitor cells and lymphoid lineage specification during hematopoiesis (By similarity). L-lysyl(4)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-homocysteine N(6)-methyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6),N(6)-dimethyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-homocysteine N(6),N(6)-dimethyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-homocysteine Component of the SET1B/COMPASS complex composed of the catalytic subunit SETD1A, WDR5, WDR82, RBBP5, ASH2L/ASH2, CXXC1/CFP1, HCFC1, DPY30 homotrimer and BOD1 (PubMed:23508102, PubMed:17355966, PubMed:18838538). 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 HCFC1 and ASH2L/ASH2 (PubMed:17998332). Interacts (via the RRM domain) with WDR82 (PubMed:17998332). Interacts (via the RRM domain) with hyperphosphorylated C-terminal domain (CTD) of RNA polymerase II large subunit (POLR2A) only in the presence of WDR82 (PubMed:17998332). Binds specifically to CTD heptad repeats phosphorylated on 'Ser-5' of each heptad. Interacts with RBM15 (PubMed:22927943). Interacts (via WIN motif) with WDR5 (PubMed:22665483, PubMed:22266653). Localizes to a largely non-overlapping set of euchromatic nuclear speckles with SETD1A, suggesting that SETD1A and SET1B each bind to a unique set of target genes. The disease is caused by variants affecting the gene represented in this entry. Belongs to the class V-like SAM-binding methyltransferase superfamily.