The assumption that triplication of the APP gene causes AD pathology in DS is in line with rare case studies of individuals with partial trisomy of chromosome 21 who have only two copies of the APP gene, where post‐mortem neuropathological examinations revealed normal age‐related changes but no evidence of AD neuropathology (Doran et al., 2017; Prasher et al., 1998). Here, APP is linked to Alzheimer disease.