text: Ligands of the sigma-2 receptor are exogenous and internalized by endocytosis, and can act as either agonists or antagonists. They can typically be classified into four groups, which are structurally related. It is not entirely understood how binding to the sigma-2 receptor occurs. Proposed models commonly include one small and one bulky hydrophobic pocket, electrostatic hydrogen interactions, and less commonly a third hydrophobic pocket.
  A study of the four groups has revealed that a basic nitrogen and at least one hydrophobic moiety is needed to bind a sigma-2 receptor. In addition, there are molecular characteristics that increase the selectivity for sigma-2 receptors, which include bulky hydrophobic regions, nitrogen-carboxylic interaction, and additional basic nitrogens.
  Since its discovery in 1990, the sigma-2 receptor has been considered an orphan receptor; however, in 2021 20S-hydroxycholesterol was identified as the putative endogenous ligand.