AP4B1 and Cognitive impairment: Interestingly, mutations in genes encoding members of the adaptor‐related protein complex 4 (AP4B1/SPG47, AP4M1/SPG50, and AP4E1/SPG51) have been associated with microcephaly and severe cognitive impairment in agreement with their role in vesicular trafficking during the development of the central nervous system [23, 24, 25, 26, 27].