The invention relates to a nucleic acid polymerase capable of producing a non-DNA nucleotide polymer from a DNA nucleotide template, said polymerase comprising amino acid sequence having at least 36% identity to the amino acid sequence of SEQ ID NO:1, wherein said amino acid sequence is mutated relative to the amino acid sequence of SEQ ID NO:1 at one or more residues of the thumb region, said residues selected from: amino acids 651 to 679 (patch 10A); wherein said amino acid sequence is mutated relative to the amino acid sequence of SEQ ID NO:1 at residue E664. In one embodiment said polymerase comprises the mutations Y409G and E664K. In one embodiment said polymerase comprises amino acid sequence corresponding to SEQ ID NO:12. The invention also relates to A nucleic acid polymerase capable of reverse transcribing a HNA nucleotide polymer into a DNA nucleotide polymer, said polymerase comprising amino acid sequence having at least 36% identity to the amino acid sequence of sEQ ID NO:1, wherein said amino acid sequence is mutated relative to the amino acid sequence of SEQ ID NO:1 at residue I521.