HEXB and Sandhoff disease: Many LSDs have a severe neurodegenerative phenotype (Platt et al., 2012, Wraith, 2004): for example, Sandhoff disease is caused by a mutation in the HEXB gene (O'Dowd et al., 1986, O'Dowd et al., 1985) which encodes the beta subunit of β-hexosaminidase and leads to developmental regression after the first few months of life, seizures, and death in early childhood (Bley et al., 2011, Smith et al., 2012b).