Table 1.
Different GPCR classification systems
| Hierarchy level | Bottom | ↔ | ↔ | ↔ | ↔ | Top | |
|---|---|---|---|---|---|---|---|
| Numerical | 1° | 2° | 3° | 4° | 5° | 6° | |
| Taxonomical | Species | Genus | Family | Order | Class | Phylum | Reference |
| NC‐IUPHAR |
Individual receptor (e.g., HTR2A) |
Subclass/subfamilies (e.g., serotonin receptors) | Class/clan/family (e.g., rhodopsin‐like) | Superfamily (GPCR) | 3 | ||
| Joseffson |
Individual receptor (e.g., HTR2A) |
Subclass (amine receptors) | Class (e.g., rhodopsin‐like) | Clade (e.g., rhodopsin‐like and secretin receptor–like | Superfamily (GPCR) | 21 | |
| GRAFS | Individual receptor (e.g., HTR2A receptor of HTR2 subgroup) | Subgroup (e.g., HTR2 subgroup of amine receptor cluster) | Branch/cluster * (e.g., amine receptor) | Group (e.g., α‐group of rhodopsin receptors) | Family/cluster * (e.g., rhodopsin‐like) | Superfamily (GPCR) | 6 |
| GRAFS/aGPCR | Individual receptor | Group* (aGPCR I–IX) | Subfamily*/family/group* (e.g., aGPCR) | Superfamily (GPCR) | 22 | ||
| Level system |
Individual receptor subtype (level 5) (e.g., HTR2A) |
Subsubfamily (level 3) (e.g., serotonin receptors) |
Subfamily (level 3) (e.g., amine receptors) |
Family (level 2) (e.g., rhodopsin‐like) |
GPCR versus non‐GPCR (level 1) | 20 | |
| GRAFS/aGPCR revised | Individual receptor/subtype | Subfamily (aGPCR I–IX) | Family (e.g., aGPCR) | Superfamily (GPCR) | 15 | ||
| New nomenclature | Individual receptor subtypes (e.g., HTR2A) | Genus of closely related subtypes (e.g., HTR2A, HTR2B, and HTR2C) | Family of different subtypes (e.g., serotonin receptors) | Order of distinct branches (e.g., amine receptors) | Class of GPCR (rhodopsin‐like) | Phylum of GPCR | This study |
Note: Different classification approaches for GPCRs. Bold terms indicate intermediate hierarchy‐level denominators, asterisks denote ambiguous use of hierarchy‐level denominators in the original publications. When we refer to published nomenclature systems, we use the terms employed therein.