Recombinant in vitro assays and mouse models have enabled the causal linking of several missense and deletion variants in GABBR1, GABBR2, AJAP1, and PIANP to a spectrum of neurodevelopmental disorders, including epileptic encephalopathy, Rett-like syndrome, global developmental delay, ID, ASD, and motor disorders. This evidence concerns the gene PIANP and atypical Rett syndrome.