Heterozygous de novo GNB1 frameshift and pathogenic splice-site and missense mutations in humans cause MRD42 (MIM 139380), an autosomal dominant neurodevelopmental disorder (also known as GNB1 encephalopathy) generally characterized by intellectual impairment, global developmental delay, hypotonia typically coupled with limb hypertonia, poor overall growth, and various manifestations of epilepsy in nearly 60% of patients (Petrovski et al., 2016; Lohmann et al., 2017; Hemati et al., 2018; Endo et al., 2020; Revah-Politi et al., 2020). The gene discussed is GNB1; the disease is epilepsy.