Abstract. The proteasome activator PA28 is an interferon--inducible complex made up of two related subunits, named PA28 and PA28, with approximately 50% amino acid sequence identity. Accumulated evidence indicates that binding of this complex to the 20S proteasome enhances the generation of classI-binding peptides. Previously, we showed that the genes coding for PA28 and PA28, designated Psme1 and Psme2, respectively, are located ~6kb apart with their 3 ends pointing toward each other on mouse Chromosome 14. In the present study, we sequenced the regions adjacent to Psme1 and Psme2. In a contiguous stretch of ~35kb, we identified six genes arranged in the following order: Cg10671-like (a gene similar to Drosophila CG10671)Psme1Cgi112 (a ubiquitously expressed gene with no known function)Psme2Flj10111 (a gene coding for a protein with two RING finger domains)Isgf3g (an interferon--inducible gene coding for an interferon-dependent, positive-acting transcription factor 3). Interestingly, the 3 untranslated region of Psme1 overlaps with that of Cgi112 by 7bp. Database analysis indicates that the corresponding human genes also overlap by up to 7bp in their 3 untranslated regions. The 5 end of the mouse, but not the human, gene coding for PA28 undergoes alternative splicing that is predicted to alter the N-terminal amino acid sequence. Comparison of the mouse sequence with a human draft sequence deposited in the NCBI database revealed that the overall organization of the region coding for the interferon--inducible proteasome activator is conserved between human and mouse.