(Molecular biology) The process of recombination (the exchange of sequence) between two DNA molecules with highly homologous (similar) sequences. This occurs naturally during the process of meiosis (the formation of gametes), where it is known as crossing over. It is also used as a useful tool for biologists, as it allows the replacement of parts of an organism's genome with another sequence - for instance, genes can be deleted (known as "knocking out" genes) or replaced with a mutated version (known as "knocking in" genes).