The introduction of a homologous DNA sequence into a specific site in the genome of a cell, either by replacement of the former sequence, i.e. sequence replacement, or by insertion into the former sequence, i.e. sequence insertion. A vector that is homologous and co-linear with a partial sequence of the targeted gene is introduced into an appropriate cell, e.g. a stem cell of some sort; it is incorporated into the genome of some of the cell's progeny and replaces the former sequence. If, however, homologous regions of the vector hybridise to the gene, and it is then incorporated into the gene by homologous recombination, the result is a disruption of the gene by insertion of the vector sequence.