Ribonuclease H1 (RNase H1) is a widespread enzyme found in a range of organisms from viruses to humans. It is capable of degrading the RNA moiety of DNA–RNA hybrids and requires a bivalent ion for activity. In contrast with most eukaryotes, which have one gene encoding RNase H1, the activity of which depends on Mg 2+ ions, Caenorhabditis eleganshas four RNase H1-related genes, and one of them has an isoform produced by alter-native splicing.