BACE1 and Alzheimer disease: In this work, we set out to establish proof‐of‐principle for safe and efficient delivery of therapeutic VHH into the CNS using next‐generation BBB‐crossing AAVs, by targeting BACE1 an important CNS target central to AD pathology (Vassar et al, 1999; Barão et al, 2016; Voytyuk et al, 2018).