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The past 20 years have seen tremendous progress in themedical treatment of cardiac diseases, a progress thatactually translated into decreased cardiovascular mor-bidity and mortality in clinical practice.1 Yet evidence accu-mulates that interventions intended to slow down the progres-sion of chronic heart diseases have approached their limit. This may be one of the reasons why the recent progress in stem cell biology has stirred so much attention. In addition, recent studies suggesting the existence of cell populations in the adult organism that, albeit at a low rate and insufficiently, endogenously replace cardiac myocytes and endothelial and smooth muscle cells have challenged the dogma of the heart as a nonregenerating organ.2–5 Such cell populations could be accessed in biopsies and principally serve as a source to replace cardiac myocytes and other cardiac cell types lost after myocardial infarction or other pathologies. Human embryonic stem cell lines, on the other hand, can nowaday