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The key role of erythropoietin ( Epo ) for colony formation by early erythroid progenitors ( burst-forming units-erythroid [ BFU-E ]) has been confirmed.
 Epo, early erythroid progenitors, erythropoietin, burst-forming units-erythroid, BFU-E
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:
 Epo, early erythroid progenitors, erythropoietin, burst-forming units-erythroid, BFU-E