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Long term culture of TPA -differentiated U-937 cells in the absence of phorbol ester for 32-36 days resulted in a process of retrodifferentiation .
 cell type: TPA -differentiated U-937 cells
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: cell line, protein, RNA, DNA, cell type 
Text: {0} 
Answer:
 cell type: TPA -differentiated U-937 cells