MRPS22, also named as C3orf5, RPMS22 and S22mt, is a component of the mitochondrial ribosome small subunit (28S) which comprises a 12S rRNA and about 30 distinct proteins. Defects in MRPS22 are the cause of combined oxidative phosphorylation deficiency type 5 (COXPD5). MRPS22 is a 360-amino-acid protein that has been found in a limited number of species, having no ortholog in fungi or prokaryotes. The exact location of MRPS22 in the three dimensional structure of the ribosome and its exact function are currently unknown. Recently, a MRPS22 defect was shown to strongly hamper assembly of the small ribosomal subunit, whereas assembly of the large subunit and part of the small subunit were only mildly affected.