BACKGROUND: Early-onset developmental and epileptic encephalopathy (DEE) is 
characterized by repeated seizures beginning within 3 months of birth and severe 
interictal epileptiform discharge, including burst suppression. This study 
assessed the utility of targeted gene panel sequencing in the genetic diagnosis 
of this disease.
MATERIALS AND METHODS: Targeted gene panel sequencing was performed in 150 early 
infantile-onset DEE patients (≤3 months of age), and we extensively reviewed 
their clinical characteristics, including therapeutic efficacy, according to 
genotype.
RESULTS: Of the early infantile-onset DEE patients, 70 were neonatal-onset DEE 
and the other 80 patients began experiencing seizures from 1 to 3 months after 
birth. There were 11 different pathogenic or likely pathogenic variants among 
34.7% (52/150) of patients with early infantile-onset DEE, in whom KCNQ2, 
STXBP1, CDKL5, and SCN1A were the major pathogenic variants. Among the 
neonatal-onset DEE patients, pathological genes were identified in 42.9% 
(30/70), indicating a significantly higher diagnostic yield than in 27.5% 
(22/80) of patients who experienced seizure onset 1 to 3 months after birth 
(p = 0.048). Among the neonatal-onset DEE group, variants in KCNQ2, STXBP1, and 
CDKL5 were detected at high frequencies, accounting for 66.7% (20/30) of the 
pathogenic or likely pathogenic variants found in this study.
CONCLUSION: Targeted gene panel sequencing demonstrated a high yield of 
pathogenic variants in the diagnosis of early-onset epileptic encephalopathy, 
especially in those with neonatal-onset DEE. Early diagnosis of early-onset 
epileptic encephalopathy may improve the prognosis of patients by earlier 
selection of appropriate treatment based on pathogenic variant.