Although genome-wide association studies (GWAS) have demonstrated that common depression is likely to result from alterations in a very large number of genes of small effect (Flint and Kendler, 2014), studies of Wolfram syndrome have identified Wfs1 as a clear example of a gene that can cause MDD in humans (Swift et al., 1990; Crawford et al., 2002; Swift and Swift, 2005). This evidence concerns the gene WFS1 and major depressive disorder.