All eight unassigned reading frames of mouse mitochondrial DNA are expressed.

Animal mitochondrial DNA contains genes for 13 potential polypeptides of significant size. Five of these genes have been assigned to distinct proteins and eight remained unassigned reading frames (URFs). Short peptides corresponding to URF protein sequences were synthesized chemically. Antibodies raised to these synthetic peptides were used to establish the existence of all eight URF proteins in mouse tissues and cells by the complementary techniques of immunoperoxidase staining, protein blotting and immunoprecipitation. Immunoperoxidase staining of thin-sectioned, freeze-substituted tissue may prove generally useful for the identification of gene products for which no formal genetic data exist. Furthermore, the ability to determine the cellular and tissue distribution of such proteins may provide the first insight into their function.