SPG7 and Atrophy: Recessive mutations in SPG7, encoding paraplegin, lead to hereditary spastic paraplegia (HSP) [13], a neurodegenerative disease affecting the long corticospinal motor axons, while dominant mutations in AFG3L2 cause spinocerebellar ataxia type 28 (SCA28) [14], associated with atrophy of the cerebellum.