The epidemiological and clinical evidence that SZ and BP disorders do not follow the rules expected for a Mendelian-genetic disorder led to the proposal that environmental insults may influence RELN gene expression by altering epigenetic regulatory mechanisms and led to the hypothesis that epigenetic factors are operative in mediating changes in the expression of RELN and other SZ candidate genes in psychotic patients (Costa et al., 2003). Here, RELN is linked to hereditary disease.