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These E1a reiteration mutants could be propagated in HeLa , A549 , and KB cells ; they were genetically stable; and they killed CREF cells at a strikingly high frequency.
 KB cells, A549, HeLa, CREF cells
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:
 KB cells, A549, HeLa, CREF cells