In their new work on the mitosome, a team led by Edmund Kunji at the MRC Dunn Unit, Jorge Tovar at the Royal Holloway University, and Martin Embley at the University of Newcastle upon Tyne provide intriguing clues to the function of this enigmatic organelle. They show that the mitosome contains a single type of a protein, called a mitochondrial carrier, that in human mitochondria exists in many different specialized versions. In humans, these diverse carriers are needed to import and export the varied chemicals required, or produced, by our complex mitochondria. The presence of a single carrier in the Entamoeba mitosome means that it must be able to do far fewer jobs than our own mitochondria. Further experiments performed by the team reveal that the Entamoeba carrier can only transport ADP and ATP, suggesting that it could fuel energy-requiring reactions within the mitosome but might not perform other functions. The work suggests that the Entamoeba mitosome may represent the simplest mitochondrion yet described, and thus it provides a model system for probing which mitochondrial functions are truly essential for eukaryotic cells.