The “power plants” of living cells, the mitochondria, probably evolved through endosymbiosis: A bacterium migrated into a primordial cell and eventually developed into an organelle that provides the cell with energy, among other things. Mitochondria produce some of the proteins they need themselves – with the help of special protein factories called mitoribosomes, which consist of RNA and proteins. Researchers in Göttingen have now provided a roadmap for how cells assemble human mitoribosomes in a modular fashion.