GRIN1-related neurodevelopmental disorder is characterized by DD/ID, muscular hypotonia, epilepsy and movement disorder. To date, fewer than 100 individuals with GRIN1-related neurodevelopmental disorder have been reported. All affected individuals known so far have developmental delay preceding intellectual disability of various degree, ranging from mild to usually severe-to-profound. Epilepsy is seen in more than half of affected individuals with numerous possible semiologies. Additional features are muscular hypotonia, movement disorders, cortical visual impairment, oculogyric crises and autism spectrum disorder. GRIN1-related neurodevelopmental disorder is inherited in an autosomal dominant or autosomal recessive manner. Most individuals reported to date with a GRIN1-related neurodevelopmental disorder whose parents have undergone molecular genetic testing have the disorder as a result of a de novo GRIN1 pathogenic variant or deletion. Some individuals have the disorder as a result of biallelic pathogenic missense or truncating variants in GRIN1.