Response: The Mitochondrial β-Signal and Protein Sorting

to differences in targeting signals in their N-terminal regions, which are completely different. This analysis is not comprehensive. Some unknown proteins that were annotated as not having a mitochondrial localization may have been missed, and there may be classes of MBOMPs without β-signals or that, like Mmm2, have β-signals distal from the C terminus. Nevertheless the process we describe finds 80% of known MBOMPs but only identifies one new candidate out of 2133 homolog clusters. We propose that the β-signal is found in all known MBOMPs and that there may be few MBOMPs remaining to be discovered. Supplemental Data Supplemental Data include one figure and one table and can be found with this article online at http://www. cell.com/supplemental/S0092-8674(08)01578-X.