Most organisms have multiple forms of RNase H; two RNases H that are dissimilar in primary amino acid sequence yet have similar structural and, presumably, a common enzymatic mechanism. To address the cellular roles of these RNases H, we have examined the effects of eliminating RNases H in unicellular organisms such as bacteria and fungi. Prokaryotic and eukaryotic cells deleted for either or both RNase H-encoding genes grow quite well but with some important differences from the parental cells. These results suggest that drugs targeted to HIV RNase H need to be examined to see if they provoke effects similar to those found in RNase H-deletion strains (e.g., increased sensitivity to DNA-damaging agents). Examinations of RNase H in mouse and human cells is currently a major focus of our research.