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Design of non - nucleoside inhibitors of HIV - 1 reverse transcriptase with improved activity towards Tyr181Cys containing variants was pursued with the assistance of free energy perturbation ( FEP ) calculations .
 Chemical: nucleoside
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: nucleoside