Two sigma receptor subtypes have been proposed, sigma1 and 2. Much of our understanding of this system is based on biochemical and pharmacological characterization of the cloned sigma1 receptor subtype (Sigma1). It has become clear that sigma receptors are not canonical receptors. Sigma1 is highly conserved among mammalian species, however, it does not share significant homology with any other mammalian protein. Although a range of structurally diverse small molecules bind Sigma1 with high affinity, and it has been associated with a broad range of signaling systems, Sigma1 itself has no known...