Abstract: The present invention provides methods for creating conditionally-immortal cell lines. These transgenic cell lines can be grown indefinitely in culture while maintaining a relatively undifferentiated stated. Upon appropriate switch signal, the cells cease replicating and differentiate much like adult cells. The switch is facilitated by the inactivation of a transforming gene, such as large T antigen. A convenient methodology for such inactivation is Cre-Lox mediated excision of the gene. Cardiac cells are provided as an example of useful a transgenic cell line.