 Embryonic stem cells from cloned embryos would also have significant advantages for biomedical research, and for drug discovery and toxicity testing. Embryonic stem cells genetically identical to the patient could provide valuable in vitro models to study disease, especially where animal models are not available, where the research cannot be done in patients themselves because it would be too invasive, or where there are too few patients to work with (as in the case of rare genetic diseases). Researchers could, for example, create large numbers of embryonic stem cells genetically identical to the patient and then experiment on these in order to understand the particular features of the disease in that person. The embryonic stem cells and their derivatives could also be used to test potential treatments. They could, for example, be used to test candidate drug therapies to predict their likely toxicity. This would avoid dangerous exposure of patients to sometimes highly experimental drugs.