SPG7 and hereditary spastic paraplegia: The m-AAA displays a hexameric ring structure that in humans can be a homo-oligomer formed by AFG3L2 only, or a hetero-oligomer formed by AFG3L2 and Paraplegin (a paralog of AFG3L2 encoded by the SPG7 gene, whose loss-of-function causes an autosomal recessive form of hereditary spastic paraplegia [12,13]).