4525
HIV-1 harboring mutations within two C/EBP sites were crippled in their ability to replicate in U937 promonocytic cells , indicating that these sites are required for replication.
 U937 promonocytic cells, C/EBP sites
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:
 U937 promonocytic cells, C/EBP sites