2042
In this review , we compare 60 crystal structures of nine GPCR subtypes ( rhodopsin , beta 2 - AR , beta 1 - AR , A ( 2a ) - AR , CXCR4 , D 3 R , H 1 R , M 2 R , M 3 R ) across four subfamilies of Class A GPCRs , and discuss mechanisms involved in receptor activation and potential allosteric binding sites across the highly variable extracellular surface of these GPCRs .
 None
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 None