Junction ribonuclease (JRNase) recognizes the transition from RNA to DNA of an RNA–DNA⁄DNA hybrid, such as an Okazaki fragment, and cleaves it, leaving a mono-ribonucleotide at the 5¢ terminus of the RNA– DNA junction. Although this JRNase activity was originally reported in calf RNase H2, some other RNases H have recently been suggested to pos-sess it.