RNases H2 are larger than H1 and usually have additional helices. The domain organization of the enzymes varies; some prokaryotic and most eukaryotic members of the H1 group have an additional small domain at the N-terminus known as the "hybrid binding domain", which facilitates binding to RNA:DNA hybrid duplexes and sometimes confers increased processivity. While all members of the H1 group and the prokaryotic members of the H2 group function as monomers, eukaryotic H2 enzymes are obligate heterotrimers. Prokaryotic HIII enzymes are members of the broader H2 group and share most structural features with H2, with the addition of an N-terminal TATA box binding domain. Retroviral RNase H domains occurring in multidomain reverse transcriptase proteins have structures closely resembling the H1 group.