Heart disease remains a leading cause of death worldwide. Owing to the limited 
regenerative capacity of heart tissue, cardiac regenerative therapy has emerged 
as an attractive approach. Direct reprogramming of human cardiac fibroblasts 
(HCFs) into cardiomyocytes may hold great potential for this purpose. We 
reported previously that induced cardiomyocyte-like cells (iCMs) can be directly 
generated from mouse cardiac fibroblasts in vitro and vivo by transduction of 
three transcription factors: Gata4, Mef2c, and Tbx5, collectively termed GMT. In 
the present study, we sought to determine whether human fibroblasts also could 
be converted to iCMs by defined factors. Our initial finding that GMT was not 
sufficient for cardiac induction in HCFs prompted us to screen for additional 
factors to promote cardiac reprogramming by analyzing multiple cardiac-specific 
gene induction with quantitative RT-PCR. The addition of Mesp1 and Myocd to GMT 
up-regulated a broader spectrum of cardiac genes in HCFs more efficiently 
compared with GMT alone. The HCFs and human dermal fibroblasts transduced with 
GMT, Mesp1, and Myocd (GMTMM) changed the cell morphology from a spindle shape 
to a rod-like or polygonal shape, expressed multiple cardiac-specific proteins, 
increased a broad range of cardiac genes and concomitantly suppressed fibroblast 
genes, and exhibited spontaneous Ca(2+) oscillations. Moreover, the cells 
matured to exhibit action potentials and contract synchronously in coculture 
with murine cardiomyocytes. A 5-ethynyl-2'-deoxyuridine assay revealed that the 
iCMs thus generated do not pass through a mitotic cell state. These findings 
demonstrate that human fibroblasts can be directly converted to iCMs by defined 
factors, which may facilitate future applications in regenerative medicine.