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Smad ubiquitin regulatory factors (Smurfs) and Arkadia regulate transforming growth factor‐β (TGF‐β) family signaling. Smurfs induce the ubiquitination and degradation of R‐Smads. Smurfs also mediate the degradation of type I receptors by associating with inhibitory‐Smads (I‐Smads) in response to ligands, leading to the repression of TGF‐β family signaling. In contrast, Arkadia enhances TGF‐β signaling by down‐regulating the negative regulators Smad7 and SnoN/c‐Ski.
