SMG8 and attention deficit-hyperactivity disorder: For instance, loss-of-function mutations in UPF2, UPF3B, SMG8 and SMG9 genes, and CNVs targeting UPF2, UPF3A, SMG6, EIF4A3, RNPS1 and RBM8A genes, have been implicated in a variety of neurodevelopmental disorders, including DD, ID, ADHD and TAR syndrome [14,15,27,92,94,95,96].