The use of targeted gene disruptions, or knockouts, has become commonplace in many basic research laboratories. In its most common application, this technique enables the researcher to disrupt expression of a specific gene product selectively. This approach has been particularly useful for studies on adrenergic receptor (AR) function, having been used to effect disruption of at least six of the nine known AR genes (1–6). As presented here, the knockout strategy is essentially a synthesis of two separate techniques. The first utilizes the phenomenon of homologous recombination, a process whereby foreign DNA (homologous at least in part to portions of the host genome) introduced into cells undergoes strand exchange and integration into the host genome at a specific locus. The second utilizes the pluripotent nature of embryonic stem (ES) cells, which can be cultured and manipulated ex vivo and reintroduced into host embryos. Engineered ES cells having undergone homologous recombination can be incorporated into such “chimeric” embryos, potentially giving rise to all cell types of the adult mouse, including germ cells. Through simple mating experiments, one can then transmit the ES cell-derived disrupted allele to progeny, and eventually intercross heterozygotes to generate mice homozygous for the disrupted allele.