A mutation within exon 17 at codon 717 of the beta-amyloid protein precursor (APP) gene is one cause of early onset familial Alzheimer's disease. Direct sequencing of exons 16 and 17 of the beta-amyloid precursor protein gene in 14 families with familial early onset Alzheimer's disease without the known pathogenic mutation (APP717) failed to reveal other mutations within the beta-amyloid sequence in this form of the disorder.