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. Part of chromatin remodeling machinery, forms H3K4me1, H3K4me2 and H3K4me3 methylation marks at active chromatin sites where transcription and DNA repair take place (By similarity). Plays an essential role in regulating the transcriptional programming of multipotent hematopoietic progenitor cells and lymphoid lineage specification during hematopoiesis (PubMed:29916805). 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. 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. Interacts with HCFC1 and ASH2L/ASH2. Interacts (via the RRM domain) with WDR82. Interacts (via the RRM domain) with hyperphosphorylated C-terminal domain (CTD) of RNA polymerase II large subunit (POLR2A) only in the presence of WDR82. Binds specifically to CTD heptad repeats phosphorylated on 'Ser-5' of each heptad. Interacts with RBM15. Interacts (via WIN motif) with WDR5. Localizes to a largely non-overlapping set of euchromatic nuclear speckles with SETD1A, suggesting that SETD1A and SETD1B each bind to a unique set of target genes. Widely expressed. During preimplantation development expressed through all stages from oocyte to blastocyst. High expression is detected in oocyte that declines to a stable level from the 8-cell stage until 8.5 dpc. Expressed in the blastocyst in both the inner cell mass and the trophectoderm. Mutant embryos show growth retardation from 7.5 dpc and die before 11.5 dpc. Belongs to the class V-like SAM-binding methyltransferase superfamily.