Site-specific and competition editing assays. (a) HEK293 cells were co-transfected with 500 ng of a plasmid expressing miR376-a2 and 500 ng of a plasmid expressing wild-type (WT) ADAR1 or ADAR1 mutants. Background editing in HEK293 cells with 500 ng of substrate plasmid is approximately 20%. As previously observed, only one monomer in an ADAR1 dimer is required to be enzymatically active, such that addition of inactive protein initially increases editing40. Editing activity is expressed as a proportion of WT ADAR1 editing activity, which is 1 (y axis). Error bars, s.e.m. ***P = 0.0005. (b) Competition between ADAR1 and inactive ADAR1 mutants. HEK293 cells were co-transfected with 200 ng of a plasmid expressing miR376-a2 and 200 ng of a plasmid expressing ADAR1 p110 in the presence of increasing amounts of a plasmid expressing a catalytically inactive form of ADAR1 (Glu912Ala)28 or Gly1007Arg. Error bars, s.e.m. **P = 0.0026.