Sigma receptors (subtypes sigma-1 and sigma-2) are a unique class of binding sites expressed throughout the mammalian body.Although the natural ligand for these sites is unknown, steroid hormones—especially progesterone—amines derived from sphingolipids, and N,N-dimethyltryptamine—all have a relatively high affinity for binding.
The sigma-1 receptor (σ1R) is a chaperone protein that resides in the endoplasmic reticulum (ER) and functions as an inter-organelle signaling modulator. Numerous biological processes are impacted by it, such as nociception, drug addiction, cardiac activity, cancer, stroke, memory, and Alzheimer's disease. Numerous human tumors exhibit overexpression of the sigma-2 (σ2R) receptor. Its status as a proliferating tumor biomarker has been confirmed.