4762
These ligands represent invaluable tools for delineating the functional roles of distinct melatonin receptor subtypes and are viable candidates for drug development .
 Chemical: melatonin
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
 Entities: melatonin 
Answer:
 Chemical: melatonin