4067
Gel-retardation experiments show that Spi-B can bind to all putative PU.1 binding sites , but do not reveal any preferred Spi-B binding site .
 PU.1, Spi-B binding site, putative PU.1 binding sites, Spi-B, Spi-B
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:
 PU.1, Spi-B binding site, putative PU.1 binding sites, Spi-B, Spi-B