Sigma receptors (σ-receptors) are protein receptors that bind ligands such as 4-PPBP, 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. Some early literature proposed a third subtype ("sigma-3") based on phenylaminotetralin (PAT) ligands, but later work showed this binding corresponded to the histamine H1 receptor; "sigma-3" is not recognized in current nomenclature.