Derivation of heart cells from human embryonic stem cells (HESCs) and understanding the underlying developmental mechanisms is the main focus of the research. Culture conditions have now been sufficiently refined that cardiomyocyte differentiation is an efficient and reproducible process. Microarray analysis of modulations in gene expression during differentiation has shown that the major known cardiac genes are upregulated but that novel genes are also expressed. Genetically marked HESCs have been produced in which expression of the green fluorescent protein marker is retained after differentiation. This has permitted unambiguous tracing of cardiomyocytes following transplantation into a mouse heart. Long term survival of the cells and integration into the host heart has been observed and the ability of these cells to restore cardiac function in mice that have undergone myocardial infarction is being investigated.