Sigma receptors (σ-receptors) are protein receptors that bind ligands such as 4-PPBP (4-phenyl-1-(4-phenylbutyl) piperidine), SA 4503 (cutamesine), ditolylguanidine, dimethyltryptamine, and siramesine. There are two subtypes, sigma-1 receptors (σ1) and sigma-2 receptors (σ2), which are classified as sigma receptors for their pharmacological similarities, even though they are evolutionarily unrelated.
The fungal protein ERG2, a C-8 sterol isomerase, falls into the same protein family as sigma-1. Both localize to the ER membrane, although sigma-1 is also reported to be a cell surface receptor. Sigma-2 is an EXPERA domain protein with a mostly intracellular (ER membrane) localization.