Mutations in the presenilin 1 (PSEN1) gene are more commonly identified as genetic causes of early-onset familial Alzheimer's disease than mutations in the amyloid precursor protein (APP) and the presenilin 2 (PSEN2) genes. More than 100 different mutations in the PSEN1 gene have been detected, and the clinical phenotypes have been described in the literature. This paper reports the case of a 38-year-old female showing early memory impairment and having a base pair mutation from guanine (G) to cytosine (C) at codon 139 of PSEN1, which leads to the substitution of a methionine with an isoleucine.