p-tau [67.3 pg/ml (42.9–91.9), DoM: +42.6% (27.1 to 58.1)] was decreased compared to patients with Alzheimer’s disease [89.0 pg/ml (70.2–111.8), P < 0.001; DoM: +90.7% (72.5 to 108.9), P < 0.001] or MCI due to Alzheimer’s disease [80.6 pg/ml (62.1–112.0), P = 0.001; DoM: +100.1% (78.1 to 122.2), P < 0.001], and elevated compared to MCI [46.8 pg/ml (34.8–64.6), P < 0.001; DoM: +8.1% (−6.0 to 22.2), P = 0.001] or HC [45.3 pg/ml (36.1–61.7), P < 0.001; DoM: (reference) ± 0.0% (−7.1 to 7.1)]. This evidence concerns the gene MAPT and early-onset autosomal dominant Alzheimer disease.