Apolipoprotein E (ApoE), encoded by the apolipoprotein E gene (APOE), plays an important role in the pathogenesis of Alzheimer's disease (AD). The APOE epsilon4 variant is strongly associated with AD. APOE promoter polymorphisms have also been reported to associate with higher AD risk. Mouse models of APOE expression have long been used to study the pathogenesis of AD. Elucidating the role of the APOE gene in AD requires understanding of how its regulation differs between mouse and human APOE genes, and how the differences influence AD risk. We compared the structure and function of both the human APOE gene promoter (hAPOEP) and mouse APOE gene promoter (mAPOEP) regions. Homology is less than 40% at 180 bp or more upstream of the two species' transcription start site (TSS, +1). Functional analysis revealed both similarities and important differences between the two sequences, significantly affected by human versus rodent cell line origin. We likewise probed nuclear extracts from several cell lines of different origins (astrocytic, glial, and neuronal) and mouse brain with specific hAPOEP and mAPOEP fragments. Each fragment shared DNA-protein interactions with the other but, notably, also bound distinct factors, demonstrated by gel shift and southwestern analyses. We determined possible identities for these distinct factors. These results suggest that regulation of mouse and human APOE genes may be sufficiently unique to justify the use of both the human APOE promoter sequence in transgenic rodent models and non-rodent AD models for studying factors involved in AD pathogenesis.