PROCESS FOR THE MANIPULATION OF NUCLEIC ACIDS

A process for engineering a host cell comprising the steps of a) integrating a first polynucleotide cassette including a first selection marker flanked by a first pair of recombination sites; b) removing the first selection marker by the action of a recombinase which recognises the first pair of recombination sites; c) integrating a second polynucleotide cassette including a second selection marker flanked by a second pair of recombination sites; and d) removing the second selection marker by the action of a recombinase which recognises the second pair of recombination sites. The first and second pairs of recombination sites have identical nucleic acid sequences within each pair, sharing 90-98% nucleic acid sequence identity. Also disclosed is a host cell genome polynucleotide comprising two recombinantly engineered regions, each adjacent to a single recombination site sharing 90-98% identity with each other and any additional recombination sites present in the host cell genome polynucleotide.