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Initiation of proliferation but not of differentiation was followed by a 20- to 25-fold increase in the nuclear level of the DNA polymerase-associated processivity factor PCNA and of the proliferation-specific transcription factor E2F1 .
 DNA polymerase-associated processivity factor PCNA, E2F1, proliferation-specific transcription factor E2F1, PCNA
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:
 DNA polymerase-associated processivity factor PCNA, E2F1, proliferation-specific transcription factor E2F1, PCNA