Because MC5R's association with schizophrenia was identified against the genetic background of a risk allele for the immunomodulatory kynurenine pathway enzyme TDO2 (Miller et al., 2009), it is most likely that the key action in this case would be the reported inhibition of IFNγ expression by MC5R (Taylor and Namba, 2001). This evidence concerns the gene MC5R and schizophrenia.