The Alzheimer Biomarker Consortium-Down Syndrome (ABC-DS) project is a large research initiative to find biomarkers to track Alzheimer disease in people with Down syndrome. Over a five-year period, researchers will collect information and blood samples from people with Down syndrome and their siblings. Participants may undergo physical exams, neurological tests, brain scans, and assessments to measure cognition, memory, and movement. Additional information will be obtained through personal and family health history, caregiver questionnaires, and genetic studies. Information gained from this initiative may help people with Alzheimer and Down syndrome, as well as people with only Alzheimer disease. Supported by grant #U19 AG068054 from the National Institute on Aging and Eunice Kennedy Shriver National Institute of Child Health and Human Development, both part of NIH.