For example, microglia showed a reduced expression of Csmd1, a complement regulatory gene linked to familial epilepsy (Naseer et al, 2016) and schizophrenia (Schizophrenia Psychiatric Genome‐Wide Association Study & C, 2011), and of the protocadherin Pcdh9, which is a risk factor for major depressive disorder (Xiao et al, 2018). This evidence concerns the gene PCDH9 and major depressive disorder.