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Therefore, we also evaluated activation of the MAP kinase extracellular signal-regulated kinase ( ERK ) 2 by the phorbol ester phorbol 12-myristate 13-acetate ( PMA ).
 ERK, MAP kinase extracellular signal-regulated kinase, MAP kinase extracellular signal-regulated kinase ( ERK ) 2
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:
 ERK, MAP kinase extracellular signal-regulated kinase, MAP kinase extracellular signal-regulated kinase ( ERK ) 2