Some prokaryotes possess an additional H2-type gene designated RNase HIII in the Roman-numeral nomenclature used for the prokaryotic genes. HIII proteins are more closely related to the H2 group by sequence identity and structural similarity, but have substrate preferences that more closely resemble H1. Unlike HI and HII, which are both widely distributed among prokaryotes, HIII is found in only a few organisms with a scattered taxonomic distribution; it is somewhat more common in archaea and is rarely or never found in the same prokaryotic genome as HI.