Mutations in the mitochondrial m-AAA protease genes cause two different neurodegenerative diseases in humans: loss of function of SPG7 is associated with an autosomal recessive form of spastic paraplegia [6], whereas missense mutations in AFG3L2 have recently been associated with SCA28 [8]. Here, AFG3L2 is linked to spinocerebellar ataxia type 28.