The mitochondrial oxidative phosphorylation system consists of a series of large complexes (such as complexes III and IV and ATP synthase), each containing many subunits. The subunits are encoded by mitochondrial or nuclear genes. Messenger RNAs that encode subunits with a nuclear origin are translated by ribosomes in the cytoplasm, and the resulting proteins are imported into mitochondria. By contrast, mRNAs of the subunits encoded by mitochondrial genes are translated by mitochondrial ribosomes in the organelle's matrix. Couvillion et al.1 report that the translation, but not the transcription, of nuclear and mitochondrial mRNAs is synchronized.