The US government has unveiled final ground rules for "ethically responsible, scientifically worthy" embryonic stem cell research studies, eligible for federal funds.
The new rules, which go into effect on Tuesday, follow President Barack Obama's March 9 executive order lifting a ban on embryonic stem cell research, an order that went into effect under his predecessor, George W. Bush.
They allow funding for research using human embryonic stem cells created by in-vitro fertilization (IVF) for reproductive purposes and no longer needed, in a departure from the Bush administration's policy.
The National Institutes of Health's (NIH) guidelines are slightly less restrictive than those outlined in a draft document released in April in that they allow the use of existing stem cell lines, in addition to new ones derived from IVF procedures.
The rules, the agency said, lay out which research is eligible for federal funding and help "ensure that NIH-funded research in this area is ethically responsible, scientifically worthy and conducted in accordance with applicable law."
Embryonic stem cell research in a laboratary in the USA.
Bush barred federal funding from supporting work on new lines of stem cells derived from human embryos in 2001, allowing research only on a small number of embryonic stem cell lines that existed at the time.
Using human embryos for scientific research, which often involves their destruction, crossed a moral barrier and urged scientists to consider alternatives, the former president argued.
In reversing the ban, the Obama administration argued that the promise of medical breakthroughs through stem cell research could not go unexplored.
Scientists say such research is key to designing treatments for diseases such as Alzheimer's, Parkinson's and diabetes.
Many researchers have been awaiting the NIH guidelines to design research projects that would be eligible for federal funding.
The NIH received some 49,000 comments from patient advocacy groups, scientists, medical groups and other interested parties before issuing the guidelines.
Stem cells -- primitive cells from early-stage embryos -- are capable of developing into almost every tissue of the body.
US scientists had lobbied hard to relax Bush's restrictions, complaining that they were falling behind their peers in other countries by not being able to conduct research on fresh stem cells derived from discarded in vitro fertilization embryos.