differentiate into one or more types of specialized cells. Embryonic stem cells is one of the sources that are being considered for the use of tissue engineering. The use of human embryonic stem cells have opened many new possibilities for tissue engineering, however, there are many hurdles that must be made before human embryonic stem cell can even be utilized. It is theorized that if embryonic stem cells can be altered to not evoke the immune response when implanted into the patient then this would be a revolutionary step in tissue engineering. Embryonic stem cells are not limited to tissue engineering. Cell replacement therapies Research has focused on differentiating ESCs into a variety of cell types for eventual use as cell replacement therapies. Some of the cell types that have or are currently being developed include cardiomyocytes, neurons, hepatocytes, bone marrow cells, islet cells and endothelial cells. However, the derivation of such cell types from ESCs is not without obstacles, therefore research has focused on overcoming these barriers. For example, studies are underway to differentiate ESCs into tissue specific cardiomyocytes and to eradicate their immature properties that distinguish them from adult cardiomyocytes. Clinical potential Researchers have differentiated ESCs into dopamine-producing cells with the hope that these neurons could be used in the treatment of Parkinson's disease. ESCs have been differentiated to natural killer cells and bone tissue. Studies involving ESCs are underway to provide an alternative treatment for diabetes. For example ESCs have been differentiated into insulin-producing cells, and researchers at Harvard University were able to produce large quantities of pancreatic beta cells from ESCs. An article published in the European Heart Journal describes a translational process of generating human embryonic stem cell-derived cardiac progenitor cells to be used in clinical trials of patients with severe heart failure. Drug discovery Besides becoming an important alternative to organ transplants, ESCs are also being used in the field of toxicology, and as cellular screens to uncover new chemical entities that can be developed as small-molecule drugs. Studies have shown that cardiomyocytes derived from ESCs are validated in vitro models to test drug responses and predict toxicity profiles. ESC derived cardiomyocytes have been shown to respond to pharmacological stimuli and hence can be used to assess cardiotoxicity such as torsades de pointes. ESC-derived hepatocytes are also useful models that could be used in the preclinical stages of drug discovery. However, the development of