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In a previous study, we demonstrated that bufalin , which is an active principle of Chinese medicine , chan'su , caused apoptosis in human leukemia U937 cells by anomalous activation of mitogen-activated protein kinase ( MAPK ) via the signaling pathway of Ras , Raf-1 , and MAPK kinase-1 .
 Raf-1, human leukemia U937 cells, MAPK, MAPK kinase-1, Ras, MAPK, mitogen-activated protein kinase
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:
 Raf-1, human leukemia U937 cells, MAPK, MAPK kinase-1, Ras, MAPK, mitogen-activated protein kinase