Mammalian mitochondrial ribosomal proteins are encoded by nuclear genes and assist in protein synthesis within the mitochondrion. Mitochondrial ribosomes （mitoribosomes） consist of a small 28S subunit and a large 39S subunit （MRPL13 is a 39S subunit protein） and have an estimated 75% protein to rRNA composition compared to prokaryotic ribosomes， where this ratio is reversed. An additional difference between mammalian mitoribosomes and prokaryotic ribosomes is that the second contain a 5S rRNA. Among various species， the proteins comprising the mitoribosome differ greatly in sequence， and sometimes in biochemical properties， which prevents easy recognition by sequence homology.
