In summary, a number of transgenic systems have been developed that enable conditional control of gene expression in vivo and in vitro. These systems have been adapted for use in respiratory epithelial cells. As in all experiments, each variable that is introduced into the system may influence the outcome and interpretation of experiments. The article by Sisson and coworkers demonstrates a potential for toxicity of the rtTA transgene when expressed under control of the CCSP promoter, a finding that may be relevant to other rtTA or tTA constructs. The study emphasizes the need for controls of all genotypes (with and without doxycycline) and for careful interpretation of experiments that are dependent on the introduction of genes in vivo and in vitro.