A cell is the basic unit of life. There are two main types of stem cells: embryonic stem cells, which can become any type of tissue or organ, and adult stem cells, which are more limited in what tissues they can form. Researchers want to study embryonic stem cells to further medical therapies, as they are adaptable and can renew themselves. However, embryonic stem cell research is politically and ethically controversial due to issues around the destruction of embryos. Former President Bush allowed only limited funding of existing embryonic stem cell lines, while his predecessor Clinton had revised funding restrictions. Religious views also differ, with some believing life begins at conception and objecting to embryonic destruction. [/SUMMARY]