GNB1-related neurodevelopmental disorder due to a pathogenic exon 6 variant NM_002074.5:c239T>A (p.Ile80Asn) inherited from an asymptomatic father with mosaicism.

GNB1-related neurodevelopmental disorder is a rare developmental encephalopathy caused by pathogenic variants in GNB1 The disorder presents with early-onset developmental delay, hypotonia, movement disorders, seizures and/or epilepsy and varied electroencephalographic (EEG) findings, which may occur even in the absence of seizures.We report a male toddler with early-onset global developmental delay, hypotonia, dysmorphic features, bilateral sensorineural hearing loss, nystagmus, transient neonatal focal seizures and emerging EEG findings. Genetic testing identified a pathogenic GNB1 missense variant NM_002074.5:c239T>A (p.Ile80Asn) inherited from an asymptomatic father with mosaicism.This case illustrates the clinical variability of GNB1 encephalopathy, expands the spectrum of reported inheritance patterns and highlights the importance of early genetic testing of infants with developmental delay and pathological EEG patterns. Recognition of the electroclinical patterns is essential for early diagnosis, genetic counselling and multidisciplinary management. The future expansion of longitudinal clinical and EEG data will be essential for prognosis and possible future therapeutic approaches.