A minority of Alzheimer disease (AD) patients begin presenting symptoms before the age of 65 years. A familial aggregation is often found in this group of early-onset AD, and, in a subset of families, the pattern of inheritance is consistent with autosomal dominant inheritance. Fully penetrant variants in amyloid precursor protein, presenilin 1 (PSEN1), and presenilin 2 are the only causative mutations reported for autosomal dominant AD. This study is to explore the PSEN1 gene mutation in a Tunisian familial Alzheimer's disease. The patient in this family showed a novel missense mutation in exon 4 of the PSEN1 gene (complementary DNA 248T > C), altering isoleucine to threonine at 83 position. Because the change occurred in conserved domains of this gene, and cosegregated with affected family member, we suggested that this change may have a mutagenic and probably pathogenic effect.