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The molecular mechanism by which ATRA increases the nuclear abundance of a functional VDR is still unknown, but our data clearly indicate that the M2 leukemic cell context is only permissive of monocytic differentiation .
 VDR, M2 leukemic cell
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:
 VDR, M2 leukemic cell