Process for the removal of selectable marker gene sequences

A process for unlabelled nucleic acid integration into the chromosome of a prokaryotic cell comprising:

• introducing a linear DNA cassette into a prokaryotic cell, wherein said linear DNA cassette comprises:
• a selectable marker gene;
• two dif-like site-specific recombinase recognition sites flanking said selectable marker gene;
• and
• two regions flanking said dif-like site-specific recombinase recognition sites which are homologous to the two regions flanking the site of integration in the chromosome of the cell;
• culturing said cell under conditions such that the linear DNA cassette is integrated into the cell chromosome by homologous recombination to produce a cell comprising said DNA cassette;
• and
• culturing said cell comprising said DNA cassette under conditions such that an endogenous site-specific recombinase encoded by the chromosome of the cell acts to excise the selectable marker gene by site-specific recombination between the dif-like site-specific recombinase recognition sites.