This initial 104 week trial is of an investigational amyloid-modifying treatment for 200 cognitively normal, autosomal- dominant early-onset AD (EOAD) mutation carriers (and 100 matched placebo-treated non-carriers) who are within 15 years of their estimated age at clinical onset. An adaptive element uses serial futility analyses and an interim analysis at 104 weeks for a study continuation decision. Outcomes include the best established brain imaging and cerebrospinal fluid (CSF) biomarkers of AD and a sensitive composite cognitive endpoint. Most participants will be presenilin 1 (PS1) E280A mutation carriers and placebo-treated non-carriers from the world’s largest, known EOAD kindred in Antioquia, Colombia. US participants are included to share opportunities and risks with the Colombian volunteers.