South Korean researchers say they have cloned a human embryo and extracted embryonic stem cells.
The experiment, the first published report of cloned human stem cells, means so-called therapeutic cloning is no longer a theory but a reality.
Supporters of medical cloning say it can transform medicine, offering tailored and highly effective treatments for diseases ranging from Parkinson's to diabetes. They say it could eventually lead to grow-your-own organ transplants.
Woo Suk Hwang
The stem cells taken from the tiny embryos, known as blastocysts, have the potential to develop into any kind of cell or tissue in the body.
"Our approach opens the door for the use of these specially developed cells in transplantation medicine," Mr Woo Suk Hwang of Seoul National University in Korea, who led the study, said in a statement.
But critics say it involves destroying a human embryo, however tiny, and is thus unethical. Writing in the journal Science, Mr Hwang and colleagues said they created the clone using eggs and cumulus cells donated by Korean women.
Cumulus cells are found in the ovaries and in some species have been found to work especially well in cloning experiments.
Scientists have cloned sheep, cattle, mice and other species but have had trouble cloning a human being. Last year a Massachusetts company, Advanced Cell Technology, said it had created a human cloned embryo but it had not grown enough to become a source of stem cells.
Stem cells are found throughout the body and are a kind of master cell. But adult stem cells are difficult to find and to work with. Many scientists believe blastocysts - stem cells taken from days-old embryos - have much greater potential. Each one, when grown correctly, can be directed to become any kind of cell or tissue at all.