Fig. 2.

Generation of targeted mutations in endogenous murine globin loci. ES cells usually derived from 129 mice (brown coat color) are electroporated with a gene-targeting construct, a generalized version of which is shown. Following drug selection and screening for proper integration of the targeting vector, the ES cells are microinjected into blastocysts normally derived from C57Bl6 mice (black coat color). Chimeric mice are identified initially based on coat color, which is a varying mixture of brown and black. If the ES cells contributed to the germ line, the targeted mutation will be inherited by the next generation of mice as indicated at the bottom of the figure. Following homologous recombination in the target ES cells, the neo cassette confers resistance to the drug G418; loss of the HSVtk cassette following homologous recombination results in ganciclovir (GANC) resistance