A method for constructing a bacterial cell comprising at least two copies of a gene of interest stably integrated into the chromosome in different positions, the method comprising the steps of: a) providing a bacterial host cell comprising at least one chromosomal copy of the gene of interest, and comprising one or more conditionally essential chromosomal gene(s) which has been altered to render the gene(s) non-funtional; b) providing a DNA construct comprising: i) an altered non-functional copy of the conditionally essential chromosomal gene(s) of step a); and ii) at least one copy of the gene of interest flanked on one side by i) and on the other side by a DNA fragment homologous to a host cell DNA sequence located on the host cell chromosome adjacent to the conditionally essential chromosomal gene(s) of step a); wherein a first recombination between the altered copy of i) and the altered conditionally essential chromomosomal gene(s) of step a) restores the conditionally essential chromosomal gene(s) to functionality and renders the cell selectable; c) introducing the DNA construct into the host cell and cultivating the cell under selective conditions that require said functional conditionally essential gene(s); and d) selecting a host cell that grows under the selective conditions of the previous step; wherein the at least one copy of the gene of interest has integrated into the host cell chromosome adjacent to the gene(s) of step a); and optionally e) repeating steps a) to d) at least once using a different conditionally essential chromosomal gene(s) in step a) in each repeat, wherein the conditionally essential chromosomal gene(s) of the host cell that is altered encodes a D-alanine racemase, or is one or more genes that are required for the host cell to grow on minimal medium supplemented with only one specific main carbon-source, or is one or more gene(s) encoding one or more polypeptide(s) involved in amino acid synthesis, and the non-functionality of the gene(s) renders the cell auxotrophic for one or more amino acid(s), and wherein restoration of the functionality of the gene(s) renders the cell prototrophic for the amino acid(s). a) providing a bacterial host cell comprising at least one chromosomal copy of the gene of interest, and comprising one or more conditionally essential chromosomal gene(s) which has been altered to render the gene(s) non-funtional; b) providing a DNA construct comprising: i) an altered non-functional copy of the conditionally essential chromosomal gene(s) of step a); and ii) at least one copy of the gene of interest flanked on one side by i) and on the other side by a DNA fragment homologous to a host cell DNA sequence located on the host cell chromosome adjacent to the conditionally essential chromosomal gene(s) of step a); wherein a first recombination between the altered copy of i) and the altered conditionally essential chromomosomal gene(s) of step a) restores the conditionally essential chromosomal gene(s) to functionality and renders the cell selectable; i) an altered non-functional copy of the conditionally essential chromosomal gene(s) of step a); and ii) at least one copy of the gene of interest flanked on one side by i) and on the other side by a DNA fragment homologous to a host cell DNA sequence located on the host cell chromosome adjacent to the conditionally essential chromosomal gene(s) of step a); wherein a first recombination between the altered copy of i) and the altered conditionally essential chromomosomal gene(s) of step a) restores the conditionally essential chromosomal gene(s) to functionality and renders the cell selectable; c) introducing the DNA construct into the host cell and cultivating the cell under selective conditions that require said functional conditionally essential gene(s); and d) selecting a host cell that grows under the selective conditions of the previous step; wherein the at least one copy of the gene of interest has integrated into the host cell chromosome adjacent to the gene(s) of step a); and optionally e) repeating steps a) to d) at least once using a different conditionally essential chromosomal gene(s) in step a) in each repeat,