Production of a homozygous mutant embryonic stem cell line (double knockout)

Under some circumstances, it may be desirable to produce a mouse cell clone in which both alleles of a desired gene are mutated. This may be because the mutation causes embryonic lethality in homozygous animals, or to test cultured cells before an animal is produced. This protocol details an easy method for obtaining homozygous cells by homologous recombination without the need for two targeting events.