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Positive selection refers to the active process of rescuing MHC -restricted thymocytes from programmed cell death .
 protein: MHC; cell type: MHC -restricted thymocytes
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: MHC, MHC -restricted thymocytes 
Answer:
 protein: MHC; cell type: MHC -restricted thymocytes