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). Responsible for H3K4me3 enriched promoters and transcriptional programming of inner mass stem cells and neuron progenitors during embryogenesis (PubMed:24550110, PubMed:29490266, PubMed:31197650). Required for H3K4me1 mark at stalled replication forks. Mediates FANCD2-dependent nucleosome remodeling and RAD51 nucleofilaments stabilization at reversed forks, protecting them from nucleolytic degradation. Does not methylate 'Lys-4' of histone H3 if the neighboring 'Lys-9' residue is already methylated (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 SET1A complex composed of the catalytic subunit SETD1A, WDR5, WDR82, RBBP5, ASH2L/ASH2, CXXC1/CFP1, HCFC1 and DPY30 homotrimer (By similarity). 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 (By similarity). Interacts with BOD1L1 (via COMPASS-Shg1 domain) at replication forks (By similarity). Interacts with HCFC1. Interacts with ASH2/ASH2L, CXXC1/CFP1 and RBBP5. 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 ZNF335. Interacts with SUPT6H (By similarity). Interacts with NAP1L1 (PubMed:29490266). Interacts (via WIN motif) with WDR5 (By similarity). Localizes to a largely non-overlapping set of euchromatic nuclear speckles with SETD1B, suggesting that SETD1A and SETD1B each bind to a unique set of target genes. High expression is detected in the oocyte that declines to a stable level from the 8-cell stage until 8.5 dpc. Mutant embryos fail to gastrulate and die before or at 7.5 dpc. Belongs to the class V-like SAM-binding methyltransferase superfamily.