Variants in CAMK2-associated genes have recently been implicated in neurodevelopmental disorders and intellectual disability. The clinical manifestations reported in patients with mutations in these genes include intellectual disability (ranging from mild to severe), global developmental delay, seizures, delayed speech, behavioral abnormalities, hypotonia, episodic ataxia, progressive cerebellar atrophy, visual impairments, and gastrointestinal issues. Phenotypic heterogeneity has been postulated. We present a child with neurodevelopmental disorder caused by a pathogenic CAMK2B variant inherited from a healthy mother. A more mildly affected sib was determined to have the same variant. Monoallelic mutations in CAMK2B in patients have previously only been reported as de novo mutations. This report adds to the clinical phenotypic spectrum of the disease and demonstrates intrafamilial variability of expression of a CAMK2B mutation.