We described a protected case with familial Alzheimer's disease, homozygous for apolipoprotein E3 (APOE3) Christchurch variant (ApoE3Ch), exhibiting low tau protein levels despite genetic predisposition to the disease due to presenilin (PSEN)1‐E280A. This evidence concerns the gene PSEN1 and early-onset autosomal dominant Alzheimer disease.