Sigma receptors were initially described as opioid receptors (Martin et al., 1976). They are now considered neither related to other types of receptor nor to each other. While several ligands used to study these receptors [e.g., di-O-tolyl guanidine (DTG) and haloperidol] have similar affinities for the sigma-1 and sigma-2 receptors, these proteins show little homology at the primary amino acid level. The sigma-1 receptor was cloned some time ago (Hanner et al., 1996; Kekuda et al., 1996), and the crystal structure of the trimer solved (Schmidt et al., 2016). The molecular identity of the sigma2 binding site has only recently been determined as TMEM97, a regulator of the sterol transporter NPC1 (Alon et al., 2017). Both proteins appear to be predominantly in the endoplasmic reticulum and, despite some evidence that progesterone and dimethyltryptamine bind the sigma-1 receptor (Fontanilla et al., 2009), are both considered orphan receptors. The sigma receptors have been implicated in a large number of apparently diverse conditions, including addiction, depression, pain, neurodegenerative conditions, cancer, and amyolateral sclerosis (among others), suggesting that their pharmacological regulation will yield useful drugs to treat several conditions.