GPC5 and early-onset autosomal dominant Alzheimer disease: Two genes required for fibrillar tau uptake, B3GALNT1 and B3GNTL1, encode glycosyltransferases (Ricci Hagman et al, 2018), and the cell surface proteoglycan, GPC5, identified in Alzheimer’s disease GWAS (Wang et al, 2020), is also required for fibrillar tau entry.