Transcriptional regulator. Regulates a number of genes involved in wing development including activation of net and bs and repression of rho and kni and controls vein-intervein patterning during wing development. Required for correct expression of a number of homeotic genes including Scr in the first leg imaginal disk and Ubx in the third leg imaginal disk and haltere disks. Required for stabilization of the histone-lysine N-methyltransferase trr and for trimethylation of 'Lys-4' of histone H3. Plays a role in maintenance of transcriptionally active chromatin through down-regulation of histone H1 hyperphosphorylation. Core component of several methyltransferase-containing complexes. Component of the SET1 complex, composed at least of the catalytic subunit Set1, wds/WDR5, Wdr82, Rbbp5, ash2, Cfp1/CXXC1, hcf and Dpy-30L1. Component of the MLL3/4 complex composed at least of the catalytic subunit trr, ash2, Rbbp5, Dpy-30L1, wds, hcf, ptip, Pa1, Utx, Lpt and Ncoa6. Interacts with hcf, sktl and trr. Accumulates on salivary gland polytene chromosomes. In larvae and pupae, expressed in imaginal disks, salivary gland and fat body cells. No expression detected in central nervous system (at protein level). Expressed in larval and pupal stages (at protein level). Expression also detected at early embryonic stages and in adult. Generally pupal-lethal with mutants showing a wide array of homeotic transformations. Adult escapers are sterile and show pattern formation abnormalities in legs, including tissue overgrowth and small supernumerary legs. In wings, the pattern formation defects observed include duplicated bristles and sockets, transformation of campaniform sensilla (a class of sensory organ) to bristles, ectopic campaniform sensilla and reduction of intervein tissue with increase of longitudinal veins and cross-vein tissue. Mutant wing imaginal disk shows ectopic expression of neur, normally expressed in all sensory organ precursors in the posterior region of the wing disk. Increased histone H1 hyperphosphorylation in polytene chromosomes. Reduced trimethylation of histone H3 'Lys-4', reduced levels of trr protein and severe defects in pupariation and metamorphosis due to a lack of activation of ecdysone-responsive genes. Intron retention.