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SMAD （mothers against decapentaplegic homologs） molecules are the core components in TGF-β signaling pathway. TGF-β binding to its receptor induces phosphorylation and activation of receptor-regulated SMADs （R-SMADs）, SMAD2 and SMAD3, which subsequently associate with their partner SMAD4 and translocate from cytoplasm to nucleus. Formation of R-SMAD–SMAD4 complexes is essential in signaling of most TGF-β family members.