MRP-S23 is one of the proteins that makes up the 55S ribosome in eukaryotes from nematodes to humans. It does not appear to carry any common motifs, either RNA binding or ribosomal protein motifs. All of the mammalian MRPs are encoded in nuclear genes that are evolving more rapidly than those encoding cytoplasmic ribosomal proteins. The MRPs are imported into mitochondria where they assemble coordinately with mitochondrially transcribed rRNAs into ribosomes that are responsible for translating the 13 mRNAs for essential proteins of the oxidative phosphorylation system. MRP-S23 is significantly up-regulated in uterine cancer cells.