Abstract: Tbx5 and Tbx3 were shown to be key determinants of regional identity in the AVCS. Various embodiments relate to increasing the amount of Tbx5 or Tbx3 in a cardiomyoctye whereby the cardiomycyte is converted to a fast conducting cardiomyocyte or a slow conducting nodal cell, respectively. Further embodiments employ the fast conducting cardiomyocytes and nodal cells in therapeutic methods and in methods for screening compounds for effect on these cell types.