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-S22 (mitochondrial 28S ribosomal protein S22), also known as S22mt, is a 360 amino acid mitochondrial ribosomal protein. Localized to the mitochondria, MRP-S22 is present in the 28S subunit of the mitoribosomes. Defects of MRP-S22 are the cause of combined oxidative phosphorylation deficiency type 5 (COXPD5). COXPD5 is an antenatal mitochondrial disease characterized by hypotonia, edema, cardiomyopathy and tubulopathy.