Developmental and epileptic encephalopathy 9 (DEE9) (MIM #300088), also known as Protocadherin 19 (PCDH19) clustering epilepsy, is an X-linked disorder caused by heterozygous pathogenic variants in the PCDH19 gene. The gene discussed is PCDH19; the disease is developmental and epileptic encephalopathy, 9.