A very small organelle, thought to be a remnant of a mitochondrion, found in certain single-celled organisms. Unlike mitochondria, mitosomes lack DNA and the apparatus for energy generation, their only apparent function being the production of iron-sulphur clusters, which are components of certain key metabolic enzymes. Mitosomes occur in several groups of anaerobic or parasitic organisms, including the protists Entamoeba histolytica and Giardia intestinalis, both intestinal parasites of mammals, and the microsporidia, which are microscopic parasitic fungi.