In 2019, an adeno-associated virus (AAV)-9-based gene therapy was approved for the treatment of infantile spinal muscular atrophy (SMA)-1 with a 1-time intravenous (IV) administration of the self-complementary (sc)-AAV9 encoding the survival motor neuron type 1 (SMN1) gene (Mendell et al., 2017). Here, SMN1 is linked to spinal muscular atrophy, type 1.