9241
The natural dominant negative HLH factor Id3 , which blocks transcriptional activity of a number of known bHLH factors , was expressed in CD34+ progenitor cells by retrovirus-mediated gene transfer.
 bHLH factors, Id3, CD34+ progenitor cells, dominant negative HLH 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:
 bHLH factors, Id3, CD34+ progenitor cells, dominant negative HLH factor