AP4B1 and hereditary spastic paraplegia: Neuropathological characterization of various non-AP-4-related spastic paraplegias has previously identified cerebellar abnormalities in a number of complex HSP subtypes.4,32,33 Histopathological examination of the cerebellar region of Ap4b1 (−/−) mice did not show any major pathological abnormalities such as atrophy, severe neuronal loss or Purkinje cell loss and the cerebellum appeared morphologically normal compared with WT mice.