In order to test the hypothesis that allelic variation within the Amyloid Precursor Protein (APP) gene influences susceptibility to common forms of Alzheimer's disease (AD) we screened the entire coding, promoter and 3' untranslated sequences of the APP gene for DNA variations in 30 unrelated patients and eight controls with probable AD by a combination of RT-PCR PCR and chemical cleavage mismatch analysis. Although we were unable to detect commonly occurring allelic variants, we were able to detect a novel mutation within the APP gene in one individual with late-onset AD. This mutation resulted in the substitution of a tryptophan residue for an arginine residue at codon 328 within exon 7 which encodes the so-called protease inhibitor domain of the 751 residue APP isoform. However, the pathological significance of this mutation is uncertain as neither this, nor any other mutation occurring within exon 7 of the APP gene was found in any of a further 102 AD patients and 86 age-matched controls. In conclusion, it is unlikely that susceptibility to AD results from commonly occurring allelic variants of the APP gene and it is even less probable that mutations within exon 7 of the APP gene are important risk factors for late-onset AD.