PNPLA6 and hereditary spastic paraplegia: The importance of NTE for neurobiology is emphasized by the observation that loss-of-function mutations in the human NTE/PNPLA6 gene are linked to several complex neurodegenerative syndromes, such as Motor neuron disease (MND), Hereditary Spastic Paraplegia 39 (SPG39), Boucher-Neuhäuser syndrome, Gordon-Holmes syndrome, cerebellar ataxia, Oliver-McFarlane syndrome and Laurence-Moon syndrome [16,17,18,19,20,21].