Defects in Chromodomain Helicase DNA-Binding Protein 2 (CHD2) are known to cause developmental and epileptic encephalopathy 94 (MIM#615369), whereas Regulator of Nonsense-Mediated mRNA Decay (UPF3B) is associated with X-linked syndromic intellectual developmental disorder 14 (MIM#300676). Here, UPF3B is linked to Lennox-Gastaut syndrome.