For example, several studies highlight a link between schizophrenia and anomalies in the expression of certain genes that are necessary for the formation of the PNNs (such as NEUROCAN, HAPLN4, or PTPR; Schizophrenia Working Group of the Psychiatric Genomics Consortium, 2014; Testa et al., 2019). This evidence concerns the gene HAPLN4 and schizophrenia.