15483
Moreover , the PEI - betaine conjugates display satisfactory DNA condensation capability ; and in the absence and presence of serum , PEI - DMAAPS23 % / pEGFP and PEI - DMAAPS55 % / pEGFP complexes exhibited remarkable gene transfection efficiencies determined by flow cytometry , which are in general several times higher than that of PEI 25K .
 PEI, PEI, DMAAPS23, PEI, DMAAPS55, PEI
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 PEI, PEI, DMAAPS23, PEI, DMAAPS55, PEI