DNA from the probands of seven Australian families with hereditary Alzheimer's disease was screened for the presence of known mutations in the amyloid precursor protein (APP) gene on chromosome 21 using single stranded conformational polymorphism (SSCP) analysis [14]. One subject was found to have a mutation causing a Val-->Ile substitution at position 717. This was confirmed by restriction enzyme digestion and sequencing. The mutation has been found in both the other affected family members available for study and in two at-risk relatives. It was not present in the only living unaffected relative who has passed the usual age of onset in this family. There is so far no evidence that apolipoprotein E (APOE) genotype influences age of onset in this family, though numbers are small. Two other families with autopsy confirmation and age of onset in the fifth decade had no APP mutation and are thought likely to have a mutation on chromosome 14 on the basis of their earlier onset age.