Mammalian mitochondrial ribosomes (mitoribosomes) are responsible for protein synthesis within the mitochondrion. The mitoribosomes are composed of a 4:1 ratio of protein to RNA, with the proteins forming two subunits, the 28S subunit and the 39S subunit. Across species, the proteins that make up the mitoribosome subunits vary greatly in sequence, preventing easy recognition by sequence homology. MRP-63 102 amino acid protein that has been identified on the intact 55S mitoribosome subunit. It is theorized that MRP-63 localizes to the subunit interface and dissociates from the 55S mitoribosome during subunit separation.