Familial dementias, which include Alzheimer disease (AD), familial British dementia (FBD), and familial Danish dementia (FDD), are caused by dominantly inherited autosomal mutations and are characterized by the production of amyloidogenic peptides, neurofibrillary tangles (NFTs) and neurodegeneration (St George-Hyslop and Petit, 2005; Garringer et al., 2009). The prevailing pathogenic theory, the “amyloid cascade hypothesis” (Hardy and Selkoe, 2002), posits that the accumulation of amyloidogenic peptides triggers tauopathy, neurodegeneration, and cognitive and behavioral changes. However, this hypothesis is yet to be validated, and causes of dementia may be multifaceted and involve other mechanisms, such as loss of function due to pathogenic mutations. Mouse models of human dementia invariably use transgenic expression systems (LaFerla and Oddo, 2005; McGowan et al., 2006; Vidal et al., 2009; Coomaraswamy et al., 2010) that do not reflect the genotypes of human disease and cannot replicate loss of function. Therefore, we generated a knock-in (KI) mouse model of FBD (FBD_KI) genetically congruous with the human disease. FBD is caused by a missense mutation at the stop codon of theBRI2gene (Vidal et al., 1999) and, like FBD patients, FBD_KImice carry this mutation in one of the two murineBri2alleles. We report that the British mutation drastically reduces expression of mature BRI2 in both KI mice and human FBD brains. This deficit is associated with severe hippocampal memory deficits in FBD_KImice. Remarkably, these animals showed no cerebral amyloidosis and tauopathy.Bri2^+/−mice present memory deficits similar to those in FBD_KIanimals. Collectively, these results indicate that the BritishBRI2mutation underlies abnormal memory due to loss of BRI2 function and independently of histopathological alterations typically evident in advanced neurodegenerative disease.