They have an estimated 75% protein to rRNA composition compared to prokaryotic ribosomes, where this ratio is reversed.

Another difference between mammalian mitoribosomes and prokaryotic ribosomes is that the latter contain a 5S rRNA.

Among different species, the proteins comprising the mitoribosome differ greatly in sequence, and sometimes in biochemical properties, which prevents easy recognition by sequence homology.

[7] Available sequence data analyses identified splice variants that differ in the 5' UTR; both transcripts encode the same protein.

"The Status, Quality, and Expansion of the NIH Full-Length cDNA Project: The Mammalian Gene Collection (MGC)".