It was in Cambridge that Dr. Cohen first successfully froze and thawed a human blastocyst for use in IVF.

While a clonal human blastocyst has been created, stem cell lines are yet to be isolated from a clonal source.

The human blastocyst comprises 70-100 cells.

Scientists often find it impossible even to extract and grow stem cells from a normal human blastocyst.

In July 2005, Hwang et al. filed a patent in the EU for the culture medium he used for human blastocysts.

Many scientists agree, saying they need to generate new lines of embryonic cells from discarded human blastocysts, or very early embryos.

"Because these cells are derived from human blastocysts there is a moral authority here, so we take these cells seriously," said Dr. Okarma, of Geron.

Ten years ago, a team from the University of Wisconsin-Madison developed a technique to get human embryonic stems cells (hES) from human blastocysts.

The creation of human embryonic stem cells has been controversial because some critics, including the Roman Catholic Church, object to destroying human blastocysts, even the surplus ones created in fertility clinics.