Process for the manipulation of nucleic acids

A host cell comprising a host cell genome polynucleotide containing a first recombinantly engineered region, a second recombinantly engineered region, a first recombination site scar and a second recombination site scar,

• wherein the first recombination site scar comprise a nucleic acid sequence of the SEQ ID NO:1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, and is adjacent to the first recombinantly engineered region in which part of the genomic polynucleotide has been engineered by the addition, deletion or replacement of a nucleic acid sequence, and
• the second recombination site scar is adjacent to the second recombinantly engineered region in which part of the genomic polynucleotide has been engineered by the addition, deletion or replacement of nucleic acid sequence;
• wherein the first recombinantly engineered region comprises deletion of a waaL gene and the second recombinantly engineered region comprises deletion of at least part of a wca colonic acid cluster and/or deletion of at least part of an rfb cluster,
• wherein the first and second recombination site scars have different polynucleotide sequences and the sequence of the second scar is less than 98% identical to SEQ ID NO:1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, and wherein the first and second recombination sites are 30-50 base pairs in length and where the first or second recombination site is a FRT site said FRT site is 48 base pairs in length.