Mitochondrial ribosome is a kind of ribosome existing in mitochondrial matrix, which is responsible for translation in mitochondria. The sedimentation coefficient skeleton of mitochondrial ribosomes is between 55s and 56s. Generally, mitochondrial ribosomes are composed of 28S ribosome subunits (small subunits) and 39S ribosome subunits (large subunits). Among these ribosomes, rRNA accounts for about 25% and ribosomal protein accounts for about 75%. The protein content of mitochondrial ribosomes is the highest. Protein in mitochondrial matrix is collectively called "mitochondrial matrix protein", including DNA polymerase, RNA polymerase, citrate synthase and enzymes in tricarboxylic acid circulating enzyme system. Most mitochondrial matrix proteins are encoded by nuclear genes. Mitochondrial matrix proteins are not necessarily expressed only in mitochondrial matrix, but can also be expressed outside mitochondria.