2248
Here , we report that a single intravenous injection of Ad containing the prolidase gene with a 6 x histidine - tag ( Ad - prolidase ) introduced high - levels of rHu prolidase in the circulation of mice which peaked on days 5 - 7 at 159 +/- 129U / mL .
 Chemical: histidine
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: histidine