RECENTLY we cloned a new member of the X11 protein family, X11L2 (gene symbol APBA3) which interacts with Alzheimer's β-amyloid precursor protein (APP) and has three protein–protein interaction domains, a phosphotyrosine interacting domain (PID) and two PDZ. Here, we report the genomic structure and mapping of the APBA3. The gene spans about 11 kb and is composed of ten coding exons and one untranslated exon. The transcription start site of APBA3 was found in a CpG island. About 1.2 kb of the 5′-flanking region was also sequenced, and its functional promoter activity was confirmed by transient transfection experiments. The APBA3 was localized by the radiation hybrid mapping to chromosome 19. Determination of the APBA3 genome structure will facilitate the linkage analysis and search for mutations in the APBA3 in patients with Alzheimer's disease.