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One noteworthy new class of agents that has yielded durable responses is programmed death ( PD ) - 1 inhibitors , which target a T - lymphocyte checkpoint and are heralding a resurgence of immunotherapy .
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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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