APP and Down syndrome: Dysregulation of endocytosis in Down syndrome has been linked to an increased dose of several endosomal pathway-related genes that map to chromosome 21, such as amyloid precursor protein (APP), synaptojanin-1 (SYNJ1), intersectin-1 (ITSN1), and regulator of calcineurin 1 (RCAN1) [23,39].