We hypothesized that schizophrenia includes several natural species in order to perform genetic studies and identify biomarkers in this huge disease type. Kraepelin was thought to consider the hypothesis of natural species because he expected the neuropathological alteration of schizophrenia when he created the concept of Dementia Praecox with progressive prognosis. We introduce natural, substantial and ideal species to discuss the reasons for the difficulty of genetic studies and drug development for schizophrenia from the point of view of the disease as a type. We detected small species from the large type and depict what schizophrenia is at the end of our manuscript. The rationale of the endogenous disease proposed by Jaspers and Schneider as a brain disorder was also considered.
 Authors' abstract