MAPT and early-onset autosomal dominant Alzheimer disease: In the healthy human brain, 3R-/4R-tau isoforms are expressed in a similar ratio, but this ratio is disturbed in Alzheimer’s disease (AD; for review, see Alavi Naini and Soussi-Yanicostas, 2015; Liu et al., 2015; Cheng and Bai, 2018).