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We previously reported the purification of human PTF , a complex of four subunits , and the molecular cloning and characterization of PTF gamma and delta subunits .
 human PTF, four subunits
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:
 human PTF, four subunits