18803
In addition , for efficient cutting , the enzymes require two cytosines 21 - 22 - nt apart and on opposite strands where one cytosine must be modified .
 Chemical: cytosines; Chemical: cytosine
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: cytosines; Chemical: cytosine