For example, studies of schizophrenia have revealed synergistic SNP-SNP interactions in dopamine receptor D4 (DRD4) [48] and in neuregulin 1 (NRG1)/v-erb-a erythroblastic leukemia viral oncogene homolog 4 (avian) (ERBB4)/AKT1 genes [49-51]. Here, DRD4 is linked to schizophrenia.