Highlighting the significant role endosomal trafficking dysfunction may play in neurological diseases comes from the pathological enlargement of early endosomes in the Alzheimer’s disease (AD) brain, and that many AD-related genes identified as risk factors by genome-wide association studies (GWAS) are involved in different stages of endosomal trafficking (e.g., PICALM, SORL1) [9, 10]. Here, PICALM is linked to Alzheimer disease.