We describe an 8-year-old Japanese boy with a de novo recurrent missense mutation in CSNK2A1, c.593A>G, that is causative of Okur–Chung neurodevelopmental syndrome. He exhibited distinctive facial features, severe growth retardation with relative macrocephaly, and friendly, hyperactive behavior. His dysmorphic features might suggest a congenital histone modification defect syndrome, such as Kleefstra, Coffin–Siris, or Rubinstein–Taybi syndromes, which are indicative of functional interactions between the casein kinase II, alpha 1 gene and histone modification factors.