4151
A major obstacle in understanding the signaling events that follow CD28 receptor ligation arises from the fact that CD28 acts as a costimulus to TCR engagement , making it difficult to assess the relative contribution of CD28 signals as distinct from those of the TCR .
 ( protein: TCR, None) , ( protein: CD28, None) , ( protein: TCR, None) , ( protein: CD28, None) , ( protein: CD28, 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: TCR, None) , ( protein: CD28, None) , ( protein: TCR, None) , ( protein: CD28, None) , ( protein: CD28, None) 