The simultaneous overexpression of several transcription factors has emerged as 
a successful strategy to convert fibroblasts into other cell types including 
pluripotent cells, neurons, and cardiomyocytes. The selection and screening of 
factors are critical, and have often involved testing a large pool of 
transcription factors, followed by successive removal of single factors. Here, 
to identify a cardiac transcription factor combination facilitating mouse 
fibroblast reprogramming into cardiomyocytes, we directly screened all triplet 
combinations of 10 candidate factors combined with a Q-PCR assay reporting 
induction of multiple cardiac-specific genes. Through this screening method the 
combination of Tbx5, Mef2c, and Myocd was identified to upregulate a broader 
spectrum of cardiac genes compared to the combination of Tbx5, Mef2c, and Gata4 
that was recently shown to induce reprogramming of fibroblasts into 
cardiomyocytes. Cells cotransduced with Tbx5, Mef2c, Myocd expressed cardiac 
contractile proteins, had cardiac-like potassium and sodium currents and action 
potentials could be elicited. In summary the alternative screening approach that 
is presented here avoided the elimination of transcription factors whose potency 
is masked in complex transcription factor mixes. Furthermore, our results point 
to the importance of verifying multiple lineage specific genes when assessing 
reprogramming.