The science of the human embryo is a focus of this year's presidential campaign in the argument over stem cells.
In 2001, President Bush lifted the complete ban on all research that destroys the embryo, announcing he would allow government money to be used in the study of human embryonic stem cells created before Aug. 9, 2001. These cells are harvested from 100-cell human embryos.
Many researchers have argued that the 22 stem cell lines this decision made available to American scientists are inadequate to aid the search for cures to diseases such as Parkinson's and macular degeneration.
They argue the current cell lines potentially are contaminated because they were raised on mouse cells, and that today's cell lines represent only a tiny slice of genetic diversity.
In addition, some argue that cloning to create stem cells may be the best way to create disease treatments.
Stem cell opponents argue that any destruction of the human embryo is taking a life, whether that embryo was made by a fertility clinic, or through cloning.
Neither candidate, and no one active in stem cell research, favors cloning to create a pregnancy.
The campaigns of both presidential candidates were asked to discuss these issues. Both responded via e-mail.
President Bush continues to oppose expanding the number of stem cell lines available, or the use of any cell lines created after Aug. 9, 2001.
"The President remains committed to fully exploring the promise and potential of stem cell research without violating ethical principles and while maintaining respect for all human life," the Bush campaign said.
To increase the number of stem cell lines available would be to sanction the destruction of more embryos, which the President views as the equivalent of human life. This year, research into existing human embryonic stem cells received $24.8 million from the National Institutes of Health. Research into adult, fetal, and umbilical cord blood cells received $190.7 million.
The President would not allow cloning to produce stem cells.
"President Bush believes all human cloning is wrong, and that advances in biomedical technology must never come at the expense of human conscience," according to the campaign. The administration supports a comprehensive law against all human cloning, a campaign spokesman wrote.
Sen. John Kerry supports a broadening of embryonic stem cell research, allowing scientists to use and create more stem cell lines. Scientists say that today's cell lines represent only a tiny slice of genetic diversity.
The Kerry campaign said that the President's ban on federal funding for stem cell lines created after Aug. 9, 2001, is "stifling research" that could cure or treat Americans suffering from numerous diseases.
"As president, John Kerry will lift the ideologically driven restrictions on stem cell research that is impeding progress ... while ensuring strict ethical oversight of the research."
Unlike the President, Mr. Kerry favors legislation that would allow stem cell lines to be created through cloning.
In this process, the nucleus of a human egg is replaced by the nucleus of an adult cell.
The egg then is stimulated to divide like an embryo, and stem cells are harvested from it when it grows to 100 cells.
Allowing cloning for stem cells, the campaign said, would "provide greater access to stem cells to conduct the important research we need."