The use of xanomeline, a partial agonist with preference for muscarinic M1 and M4 receptors (CHRM1/CHRM4)1 (Bymaster et al., 1997), to treat schizophrenia was underpinned by a hypothesis reflecting many lines of evidence arguing that activating CHRM1 and CHRM4 would be a mechanism to reduce the severity of the symptoms of the disorder (Bymaster et al., 1999; Dean et al., 2003). This evidence concerns the gene CHRM4 and schizophrenia.