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These studies demonstrate that a single cytokine can activate different combinations of Stat proteins under different physiological conditions , and also indicate two mechanisms by which distinct cytokines can activate the same Stat protein .
 ( protein: cytokine, None) , ( protein: cytokines, None) , ( protein: Stat protein, None) , ( protein: Stat proteins, None) 
Locate the role in the text that participated in the event based on the event type and return it in the event list. 
 Option: Event type: RNA,cell type,protein,cell line,DNA, Arguments type: . 
Text: {0} 
Answer:
 ( protein: cytokine, None) , ( protein: cytokines, None) , ( protein: Stat protein, None) , ( protein: Stat proteins, None) 