Human RNase H1 mutants

A human RNase H1 polypeptide which comprises a mutation compared to wild type human RNase H1, wherein said mutation produces a mutant version of human RNase H1 polypeptide that consists of a deletion mutation of region I, a deletion mutation of region II, a deletion mutation of region I and region II, or a lysine-to-alanine substitution at amino acids 226 and 227 of SEQ ID NO:

• 3, and wherein said mutant version retains detectable cleavage activity for the RNA strand of an RNA/DNA duplex.