The invention relates to a method for modification of a host cell at a target locus, which method comprises: providing a host cell comprising, at a first locus, at least two site-specific recombination sites and a nucleic acid having an essential function or encoding a product having an essential function; introducing into the host cell, at the target locus, a further nucleic acid having the essential function or encoding for a product having the essential function; and carrying out recombination at the first locus via the at least two site-specific recombination sites, so that the nucleic acid having an essential function or encoding a product having an essential function is rendered non-functional, thereby to modify the host cell at the target locus. The invention also relates to a cell obtainable by a method of the invention.