E3 ubiqutin ligases RNF111 (Arkadia) (Levy et al. 2007, Nagano et al. 2007) and SMURF2 (Bonni et al. 2001) ubiquitinate SKI/SKIL transcriptional repressors bound to activated SMAD2/3. The role of RNF111 was inferred from experiments that used recombinant mouse RNF111 and endogenous human SMADs and SKI/SKIL. The role of SMURF2 was inferred from experiments involving human proteins only.