Histone methyltransferase that methylates 'Lys-4' of histone H3 (H3K4me) (PubMed:12408863). H3K4me represents a specific tag for epigenetic transcriptional activation (PubMed:12408863). Functions in segment determination through interaction with genes of bithorax (BX-C) and antennapedia (ANT-C) complexes (PubMed:2107543, PubMed:7958911). Acts as an activator of BX-C (PubMed:7924996). Involved in the very early regulation of homeotic genes expressed only in the posterior region of the embryo (PubMed:7924996, PubMed:25310983). Also has auto-methylation activity on Cys-3641 (PubMed:24235145). L-lysyl(9)-[histone H3] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(9)-[histone H3] + 3 S-adenosyl-L-homocysteine L-cysteinyl-[protein] + S-adenosyl-L-methionine = H(+) + S-adenosyl-L-homocysteine + S-methyl-L-cysteinyl-[protein] Interacts (via SET domain) with ash1 (via SET domain) (PubMed:10454589, PubMed:10656681). Interacts with Nup98 (PubMed:25310983). Binds to 16 discrete sites on larval salivary gland polytene chromosomes. Maternal isoforms are expressed in syncytial blastoderm, confined to the ventral region fated to become mesoderm. An additional broad domain of expression arises during cellularization and is quickly resolved into four pair-rule-like stripes in the posterior half of the embryo. Expressed both maternally and zygotically. RNAi-mediated knock-down in the larva results in decreased expression of Hox genes such as Ubx and Antp. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily.