The proteasome is a large multicatalytic proteinase that plays a role in the generation of peptides for presentation by major histocompatibility complex class I molecules. The 20S proteolytic core of mammalian proteasomes is assembled from a group of 17 protein subunits that generate a distinctive pattern of spots upon two-dimensional gel electrophoresis. The genes for most of these subunits have been cloned from humans and rats. We isolated cDNA clones for the mouse orthologues of ten of the subunits [PSMA1 (C2), PSMA2 (C3), PSMA3 (C8), PSMA4 (C9), PSMA5 (ZETA), PSMA6 (IOTA), PSMA7 (C6-I), PSMB2 (C7-I), PSMB3 (C10-II), and PSMB5 (X)] to complete the cloning of all of the mouse subunits. Using antisera raised against these subunits or their orthologues, we verified the identity of these proteins by two-dimensional NEPHGE-PAGE.