This fall funding for embryonic stem cell research once again faces uncertainty. In August a federal district court judge blocked the use of federal funds for any project that would destroy embryos. In September a higher court restored funding temporarily, while it considers an appeal by the Justice Department. We offer a guide to the facts behind the controversy:

Where do the embryos used for stem cell lines come from?
All stem cell lines come from discarded IVF embryos. Currently there are around 400,000 embryos stored at fertility clinics around the country.

How many stem cell lines are there?
A stem cell line is a family of constantly dividing cells produced from a group of parent cells that were harvested from a single embryo. M. William Lensch, a scientist at the Harvard Stem Cell Institute, estimates there are 800 lines around the world.

Why isn’t this enough?
In theory, one stem cell line can supply an infinite number of researchers indefinitely. But despite their omnipotent potential, embryonic stem cells are a finicky lot. Some have a tendency to grow into liver cells, others into blood, and others into nerve, pancreas or heart tissue. Sometimes the differences are caused by known factors, such as embryo age or protein contamination, but more frequently they are not understood. “For some projects, existing lines work very well, but for others, not at all,” Lensch says.

Why aren’t more unused embryos made into cell lines?
Whereas some 60 percent of infertility patients would like to donate their unused embryos to research, a dearth of funding and an uncertain regulatory environment have muddied the process. “Everything is at a stand­still right now,” says Elena Gates, director of the IVF tissue bank at the University of California, San Francisco.