AP4B1 and hereditary spastic paraplegia: Adapter protein complex 4 (AP-4)-related hereditary spastic paraplegia (AP-4-HSP), which comprises AP4B1-associated SPG47 (OMIM #614066), AP4M1-associated SPG50 (OMIM #612936), AP4E1-associated SPG51 (OMIM #613744) and AP4S1-associated SPG52 (OMIM #614067), is a rare but prototypical form of childhood-onset complex hereditary spastic paraplegia (HSP) and an important genetic mimic of cerebral palsy7,8.