This includes biallelic loss-of-function variants in PCDH12 and PCDH15, which were identified as cause of diencephalic–mesencephalic junction dysplasia syndrome type 1 (DMJDS1), Usher syndrome type 1F, and nonsyndromic hearing loss, respectively, as well as PCDH19, in which over 100 different missense and nonsense variants have been reported to underlie X-linked developmental and epileptic encephalopathies 9 (DEE9) highlighting the importance of cell–cell communication via PCDH19 at the early stages of brain development [24–26, 37]. This evidence concerns the gene PCDH19 and developmental and epileptic encephalopathy, 9.