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In fetal liver of PU.1-/- mice , which lack lymphoid and myeloid cells , expression of Btk was reduced two- to threefold but not abolished.
 protein: Btk
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: cell line, protein, RNA, DNA, cell type 
Text: {0} 
 Entities: Btk 
Answer:
 protein: Btk