Background: Neuronal ceroid lipofuscinoses (NCLs), commonly known as Batten disease, are a group of rare inherited disorders with overlapping symptoms and pathology causing a progressive, terminal neurodegenerative condition that differ from each other in severity and age of onset. They mainly affect children but occur in all ages and ethnicities worldwide causing a debilitating combination of dementia, epileptic seizures, motor decline, vision loss, profound disability and early death. Currently there is no cure. There are more than 660 known mutations across the 13 CLN genes (CLN1-8, CLN10-14) which encode proteins involved in lysosome function and regulate a variety of cellular processes such as lipid metabolism and autophagy. Most treatments target the symptoms rather than the cause except for cerliponase alfa, an enzyme replacement therapy (ERT) approved for treating the loss of CLN2 in the brain. Gene therapies are under investigation in a number of the Batten disease subtypes. An individualised antisense oligonucleotide (ASO) called Milasen, was developed for a child with an unusual mutation in the CLN7 gene and went from concept to clinic in 10 months. "Project Butterfly" is a collaborative effort to develop a personalised ASO (Zebronkysen) therapy for two children with a rare CLN3 mutation. Recently, Tern Therapeutics have presented positive 12-month data from dose escalation cohorts in an ocular gene therapy trial for CLN2 disease.