13785
Indeed, USA-derived PC2 was found to act synergistically with PC4 in reproducing the function of intact USA in the assay system .
 ( protein: USA-derived PC2, None) , ( protein: PC4, 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: USA-derived PC2, None) , ( protein: PC4, None) 