Stem cells are how we all begin: undifferentiated cells that go on to develop into any of the more than 200 types of cell the adult human body holds.

Few quarrel with predictions of the awesome potential that stem cell research holds. One day, scientists say, stem cells may be used to replace or repair damaged cells, and have the potential to drastically change the treatment of conditions like cancer, Alzheimer's and Parkinson's disease and even paralysis. But the divisions over how to conduct that research have been deep and bitter, and have most recently played out in the courts. Most research has been conducted on embryonic stem cell lines -- cultures of cells derived from four- or five-day-old embryos, or fertilized cells. Opponents of embryonic stem cell research, which often uses embryos discarded by fertility clinics, want it to be severely restricted or banned outright as inhumane.

The most important legislation relating to stem cell research is known as the Dickey-Wicker amendment, which first became law in 1996, and has been renewed by Congress every year since. It specifically bans the use of tax dollars to create human embryos - a practice that is routine in private fertility clinics - or for research in which embryos are destroyed, discarded or knowingly subjected to risk of injury.

For a time, the ban stood in the way of taxpayer-financed embryonic stem cell research, because embryos are destroyed when stem cells are extracted from them. But in August 2001, in a careful compromise, President George W. Bush opened the door a tiny crack, by ordering that tax dollars could be used for studies on a small number of lines, or colonies, of stem cells already extracted from embryos -- so long as federal researchers did not do the extraction themselves.

Congress continued to be inundated with calls from people suffering from diseases for which stem cells research might be the only hope, and from equally vehement opponents. In 2006, the Republican-controlled Congress passed a bill to expand research. In response, Mr. Bush issued the first veto of his presidency. In 2007, Congress, now in Democratic hands, passed a similar bill by a larger margin, but still not by enough to override Mr. Bush's veto.

On March 9, 2009, President Obama issued an excutive order rescinding the limits set by Mr. Bush and making clear that the government supported stem-cell research.

In August 2010, Chief Judge Royce C. Lamberth of Federal District Court blocked Mr. Obama's excutive order, saying it violated a ban on federal money being used to destroy embryos.

In May 2011, a federal appeals panel voted 2 to 1 to overturn Judge Lambert's ruling. The appeals court said that because the law is written in the present tense, ¡°it does not extend to past actions,¡± meaning that research on existing lines of stem cells could continue. The ruling, however is just the latest in what is bound to be a lengthy legal battle.