12609
Thrombopoietin ( TPO ), the primary physiological regulator of platelet production , was initially thought to be a lineage-specific factor acting predominantly on megakaryocytopoiesis .
 TPO, Thrombopoietin
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:
 TPO, Thrombopoietin