2553
Using in vitro footprinting and gel shift assays , we have identified four distinct binding sites for nuclear proteins within this positive regulatory element .
 nuclear proteins, positive regulatory element, binding 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:
 nuclear proteins, positive regulatory element, binding sites