[18F]GTP-1

This substudy will evaluate the effect of crenezumab on the longitudinal tau burden in a subgroup of preclinical Presenilin1 (PSEN1) E280A mutation carriers and non-carriers, who were enrolled in study NCT01998841 (GN28352). Participants will receive up to three intravenous (IV) injections of [^18F] Genentech Tau Probe 1 (GTP1) and will undergo a tau positron emission tomography (PET) scan after each IV injection of [18^F]GTP1. The purpose of this substudy is to increase the understanding of disease progression in the preclinical stage of familial Alzheimer's Disease (AD).