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Smad6 and Smad7 prevent ligand-induced activation of signal-transducing Smad proteins in the transforming growth factor-beta family .
 transforming growth factor-beta family, signal-transducing Smad proteins, Smad6, Smad7
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: cell line, protein, RNA, DNA, cell type 
Text: {0} 
Answer:
 transforming growth factor-beta family, signal-transducing Smad proteins, Smad6, Smad7