Ribonuclease H (RNase H), first observed in extracts of calf thymus, is an apparently ubiquitous enzyme activity (1 ), which catalyzes the hydrolysis of RNA in RNA-DNA heteroduplexes (2 ). The two main classes (Type I and II) of eukaryotic RNase H have different biochemical requirements for maximal activity. A nuclear localization has been proposed (3 ,4 ), although different subnuclear fractions for the separate types have been suggested (seeref.5 ). The proposed physiological roles, DNA duplication (class I), and RNA replication (class II), are both nuclear processes (seerefs.5 and 6 ).
