9553
Notably, the lack of pRB in these U937-derived clones renders these p53-null cells highly resistant to apoptosis induced by serum withdrawal , calphostin C , and ceramide .
 pRB, p53-null cells, U937-derived clones
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:
 pRB, p53-null cells, U937-derived clones