Autosomal dominant amyloid precursor protein (APP) mutations in familial Alzheimer's disease accelerate the amyloid beta (Aβ) pathology. Here we describe Japanese siblings with a new N-terminal mutation (a heterogeneous c.674T>C, p.Val225Ala) of the APP gene, developing a progressive dementia at 57 years and Aβ and tau pathologies in cerebrospinal fluid studies. However, the brother and sister showed different clinical and neuroimaging features, suggesting different Aβ pathologies for each sibling.