↵2-a Each main receptor structure class is subdivided as outlined in Tables 3-6. If the subclass is not known definitively, the main class only will be identified as 1.0, 2.0, etc. These classes are provisional, and additional classes and subclasses may need to be added as knowledge increases. Literature references on receptors in many of the subclasses listed can be found in The IUPHAR Compendium of Receptor Characterization and Classification.