There are two main approaches to using stem cells to fight human diseases: develop stem cells to produce therapeutic replacement cells and study stem cells as a model for understanding the biology of a disease. Significant progress has been made in producing stem cell lines that, for example, participate in the regeneration of damaged nervous tissue. Many human diseases, such as juvenile diabetes (type 1 diabetes), involve malfunctioning genes and environmental triggers. Usually, a specific type of cell is primarily affected by the disease, and the cellular dysfunction produces the symptoms. In juvenile diabetes, the insulin-producing islet cells of the pancreas are destroyed. Insulin is critical to the proper regulation of sugar by the body, and its absence causes the severe condition called diabetes. Researchers want to coax embryonic stem cells into becoming healthy insulin-producing cells. These cells might then be transplanted into people with diabetes to produce the insulin they lack. Researchers are also interested in producing stem cells that malfunction exactly like the diseased cells in order to understand fundamental aspects of the disease and also to test treatments.