6851
In 1953 , Billingham , Brent , and Medawar demonstrated that murine neonatal chimeras prepared by infusion of donor - derived hematopoietic cells exhibited donor - specific tolerance to skin allografts .
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Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
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