RNAi knockdown of lid through expression of liddsRNA.Sym.Scer\UAS under the control of Scer\GAL4Act5C.PU is semi-lethal. These flies also show an elevated frequency of duplicated thoracic macrochaetae. Total H3K4 trimethylation - but not mono- or dimethylation - is markedly enhanced in liddsRNA.Sym.Scer\UAS knockdown flies and larvae.