Familial Alzheimer's disease (AD) is an autosomal dominant disorder characterized by memory impairment and multiple cognitive deficits which occurs in mid to late life. Early onset AD has been associated with mutations in three genes, of which presenilin 1 (PS1) mutations are the most frequent. We sequenced the open reading frame from genomic DNA of a series of 21 early onset AD (AD3) UK families in which there were at least two affected individuals in two or more generations with a diagnosis of probable or definite AD. We found PS1 mutations in six of these families with no sequence variation in the remaining 15. The six families contained between them five different mutations of which two, I143F and P436S, have not been found elsewhere. I143F shows incomplete penetration within the affected family. P436S is the most carboxy-terminal presenilin 1 mutation reported to date.