Abstract <p>Transduced cardiomyocyte-based constructs provide a promising platform for in vitro modeling cardiac disease and a source of cell products for treating cardiac diseases and, particularly, conduction system disorders. Neonatal rat cardiomyocyte cultures were transduced with a vector carrying a gene for the TBX5 transcription factor, which is involved in controlling the bioelectrical properties of cardiomyocytes. TBX5 overexpression was found to increase the number of functionally active cardiomyocyte clusters in a culture, to enhance their morphological complexity, to stimulate the establishment of a mature pattern of calcium dynamics, to synchronize spontaneous Ca²⁺ transients, and to facilitate the integration of cell clusters. The findings demonstrate that TBX5 overexpression directs cultured cardiomyocytes toward a phenotype resembling that of the native cardiac conduction system.</p>