Neurodevelopmental disorders are a large group of disorders that affect ~ 3% of children and represent a serious health problem worldwide. Their etiology is multifactorial and includes genetic, epigenetic, and environmental causes. Mounting evidence shows the importance of genetic causes, especially genes involved in the central nervous system development. As recently discovered, the KMT5B gene is related to abnormal activities of the enzymes that regulate histone activity and gene expression during brain development. Pathogenic KMT5B gene variants lead to autosomal dominant, intellectual developmental disorder 51 (OMIM # 617788). Also, reports on patients with additional features suggest that the KMT5B gene alterations lead to multisystem involvement. Here, we report on a male patient with a severe neurodevelopmental disorder caused by a novel KMT5B gene variant inherited from his mother. The patient had severe intellectual disability, absent speech, marked autistic behavior, attention deficit hyperactivity disorder, and different clinical features, including thoracic scoliosis, dysmorphic facial features, and tall stature. In contrast, his mother, with the same KMT5B variant, had mild intellectual disability and some autistic traits (stereotype hand movement). We elucidated pathogenetic mechanisms that could influence phenotype characteristics. Our findings emphasize the importance of a comprehensive clinical and molecular approach to these patients in order to provide optimal health care.