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These results suggest that PU.1 plays a major role in macrophage gene regulation and development by directing the expression of a receptor for a key macrophage growth factor .
 PU.1, key macrophage growth factor
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:
 PU.1, key macrophage growth factor