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The βA4-amyloid protein precursor (APP) is a transmembrane glycoprotein that is the source of the characteristic βA4-amyloid deposits of Alzheimer brains. It exists in eight isoforms generated by alternative splicing of exons 7, 8 and 15, of which the L-APP mRNAs lacking exon 15 are significantly expressed in non-neuronal cells and tissues, but not in neurones. Recently, it was shown that APP is a member of a multigene family of which the amyloid precursor-like protein 2 (APLP2) is the nearest relative. Analysis of APLP2 expression revealed regulated alternative splicing of the Kunitz protease inhibitor domain (KPI, homologous to exon 7 of APP) and a non-homologous insert of 12 amino acids on the NH2-terminal side of the transmembrane domain. While expression of the KPI encoding exon of APLP2 is abundant in neurones and thus differs from APP, L-APLP2 mRNA isoforms lacking the latter, non-homologous insert show a tissue-specific expression pattern similar to that of exon 15 of APP. Comparison of alternatively spliced APP and APLP2 mRNA isoforms in rat brain regions from early post-natal and adult rats revealed significantly higher relative amounts of KPI-encoding APP isoforms in the adult rat brain and an even more pronounced augmentation of L-APP mRNAs. Both effects were not observed for APLP2. This indicates an APP-specific age-associated regulation pattern within the APP gene family which has intriguing implications for the development of Alzheimer's disease in humans.