Genetically modified mice by means of homologous recombination in embryonic stem (ES) cells are generated by injection of manipulated ES cells into recipient blastocysts. The injected blastocysts, following reintroduction into recipient foster mice, will produce chimeric mice in which the manipulated ES cells populate the germline and transmit the induced mutation to the offspring. Crossing of the chimeras' offspring bearing the targeted mutation in heterozygosis will ultimately produce mice homozygous for the specific genetic mutation. Here we describe the steps and procedures required to generate the chimeric mice leading to the transfer of a genetic mutation to the mouse germline.