Powerful new approaches for the identification and sequencing of novel cDNAs have produced a backlog of proteins seeking functions. Traditional approaches for characterizing protein function (e.g., blocking monoclonal antibodies and heterologous expression) have significant limitations, especially in identifying the roles specific proteins play in vivo. An alternative approach is to engineer mutations in the protein of interest that abolish its function and that also inhibit the function of simultaneously expressed wild-type protein (dominant negative mutations). This approach has wide application to the study of a number of different kinds of proteins but tends to be most effective for proteins that need to assemble into multimers to be functional. Dominant negative mutants have already provided insights into the molecular mechanisms of action of a number of protein families, including hormone receptors, oncogenes, and growth factor receptors, and have been identified as the cause of at least a few autosomal dominant diseases. Expression of dominant negative mutants under the control of highly active lung cell-specific promoters holds great promise for the study of the roles specific proteins and protein families play in lung development, health, and disease.