A bacterial host cell is disclosed including at least two copies of an amplification unit in its genome, the amplification unit including: i) at least one copy of a gene of interest, and ii) an expressible conditionally essential gene, wherein the conditionally essential gene is either promoterless or transcribed from a heterologous promoter having an activity substantially lower than the endogenous promoter of the conditionally essential gene, and wherein the conditionally essential gene if not functional would render the cell auxotrophic for at least one specific substance or unable to utilize one or more specific sole carbon source; methods for producing a protein using the cell of the invention, and methods for constructing the cell of the invention.