Why haven't we cloned a human yet?

In 1996, Dolly the sheep made headlines around the world after becoming the first mammal to be successfully cloned from an adult cell.

Some people suggested that human clones could play a role in eradicating genetic diseases, while others considered that the cloning process could.

Some people suggested that human clones could play a role in eradicating genetic diseases, while others considered that the cloning process could.

Eventually, eliminate birth defects (despite research by a group of French scientists in 1999 finding that cloning may actually increase the risk of birth defects).

There have been various claims — all unfounded, it is important to add — of successful human cloning progams since the success of Dolly.

In 2002, Brigitte Boisselier, a French chemist and devout supporter of Raëlism — a UFO religion based on the idea that aliens created humanity — claimed that she and a team of scientists had successfully delivered the first cloned human, whom she named Eve.

However, Boisselier was unwilling — or indeed unable — to provide any evidence, and so it is widely believed to be a hoax. 

So why, almost 30 years on from Dolly, haven't humans been cloned yet? Is it primarily for ethical reasons, are there technological barriers, or is it simply not worth doing? 

"Cloning" is a broad term, given it can be used to describe a range of processes and approaches,

but the aim is always to produce "genetically identical copies of a biological entity," according to the National Human Genome Research Institute (NHGRI)

Any attempted human cloning would most likely utilize "reproductive cloning" techniques — an approach in which a "mature somatic cell," most probably a skin cell, would be used, according to NHGRI. 

The DNA extracted from this cell would be placed into the egg cell of a donor that has "had its own DNA-containing nucleus removed."

The egg would then begin to develop in a test tube before being "implanted into the womb of an adult female," according to NHGRI.