<p>This entry represents the N-terminal domain of RNase HI, which has a 3-layer alpha/beta/alpha structure [<cite idref="PUB00011751"/>]. This domain is lacking in retroviral and prokaryotic enzymes, but shows a striking structural similarity to the ribosomal protein L9 N-terminal domain, and may function as a regulatory RNA-binding module. However, the topology of this domain differs from structures of known RNA binding domains such as the double-stranded RNA binding domain (dsRBD), the hnRNP K homology (KH) domain and the RNP motif. Eukaryotic RNases HI possess either one or two copies of this small N-terminal domain, in addition to the well-conserved catalytic RNase H domain. RNase HI belongs to the family of ribonuclease H enzymes that recognise RNA:DNA hybrids and degrade the RNA component. </p>