Genetic differences associated with ethnic background, as well as gender effects, the inherent phenotypic heterogeneity of schizophrenia, and the presence of epistatic or epigenetic interactions are rightly considered among the first-line putative sources of variation that should require a dedicated control; however, their influence on the putative associations between the DAO, DAOA, PPP3CC, and DTNBP1 genes and schizophrenia unfortunately remains largely undetermined to date. This evidence concerns the gene DAOA and schizophrenia.