Fig. 1

Tree Diagram of advanced search in PubMed database. “Schizophrenia” and “Bipolar disorder” were used as Medical Subject Heading (MeSH) main terms while “DNA methylation” was used as simple term. Branches represents the percentage of published reports when these two keywords were combined. The last branch represent the percentage of published reports in first quartile according to impact factors of the corresponding year of publication (Journal Citation Report—Thompson Reuters). Only data from the last 5 years were included