Two recent developments suggest a route to predetermined alterations in mammalian germlines. These are, first, the characterization of mouse embryonic stem (ES) cells1 that can still enter the germline after genetic manipulation in culture2,3 and second, the demonstration that homologous recombination between a native target chromosomal gene and exogenous DNA can be used in culture to modify specifically the target locus4. We here use gene targetting functionally to correct the mutant hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene in the ES cell line which has previously been isolated and used to produce an HPRT-deficient mouse5. This modification of a chosen gene in pluripotent ES cells demonstrates the feasibility of this route to manipulating mammalian genomes in predetermined ways.