Genetics is a powerful tool for studying the function of different gene products as well as the physiological consequences of a normal or aberrant polypeptide. Until recently, the most powerful genetic applications have been restricted to relatively simple organisms whose genomes are more easily manipulated. For a number of years, in lower organisms, it has been possible to create defined genetic changes that are targeted to a specific locus or even to a defined codon or transcriptional regulatory sequence. With the development of gene targeting using embryonic stem cells derived from the preimplantation blastocyst of a mouse, it has become possible to extend these experiments to a mammalian system. Via homologous recombination, one can ablate, or "knock out," a defined genetic locus or mutate a particular set of nucleotides that encodes a peptide domain of interest. These techniques, when applied to genes that underlie normal cardiovascular function, promise to define the exact role(s) different proteins play during the development, growth, and maintenance of the heart. The ability to generate defined animal models of human disease in which the primary genetic defect is known should lead to fundamental advances in the study of the normal and failing heart.