Several mutations in the beta amyloid precursor gene cause autosomal dominant Alzheimer's Disease in a number of kindreds. Among them, the Tottori mutation produces beta amyloid peptides with the D7N substitution at the peptide N terminus. This was reported to accelerate the kinetics of oligomers formation which act as fibril seeds and are more toxic to cultured neuronal cells. In vitro analysis using beta-amyloid 1 to 40-based mutant peptide reveals that the D7N mutation does not accelerate the nucleation phase but selectively promotes the elongation phase of amyloid fibril formation. The levels of protofibrils generated from D7N beta-amyloid were markedly inhibited despite enhanced fibril formation.