Fig. 3. Scheme of biallelic mutagenesis achieved by combining Cre/loxP-mediated inter-chromosomal recombination with chemical mutagenesis. ES cells bearing Cre, two loxP sites and one copy of the mutant neo gene are mutated by chemical mutagens. After selection of cells by high-dose G418, ES cells with biallelic mutations on the chromosomes bearing the neo gene can be expected to be obtained. ‘x’ indicates the mutation introduced by chemical mutagens.