Scientists say the new cloning technologies can be made much more efficient if researchers refine their techniques.

Their optimism follows new experiments on cloned mice.

Currently, most cloning experiments fail. The technique involves the transfer of genetic material from adult or embryonic cells into empty eggs. These eggs are then activated and implanted into surrogate mothers that carry the developing embryos through to birth.

But researchers are lucky if they get this far. Very few embryos survive long enough in the lab to be transferred. Fewer still are carried to term to produce living animals.

Dolly the sheep, the most famous of all cloned animals, was the one success in 277 attempts. Low survival rates have hampered all cloning projects including those on cows, goats and mice. And no-one has yet managed to clone a pig.

Genetic make-up

Now, two studies of mice have suggested ways to increase the numbers of viable cloned offspring.

Rudolph Jaenisch and colleagues at the Whitehead Institute for Biomedical Research in Massachusetts, US, have shown that the genetic make-up of the donor cells has a profound impact on the efficiency of the cloning.

The researchers injected the nuclei from two genetically-distinct strains of cells into recipient eggs and found that they differed in their ability to produce viable offspring. The scientists say this shows that some strains of animals will be easier to clone than others.

Writing in the journal Nature Genetics, the Whitehead team also say they achieved their best results when the donor cells were "fresh" rather than being kept for some time in culture before their nuclei were transferred.

Right moment

A study by a second group of scientists found that choosing the right moment to activate an egg injected with genetic material can also improve efficiency.

Teruhiko Wakayama and colleagues at the Rockefeller University in New York treated their eggs with a chemical, strontium chloride, to trigger embryonic development.

They did this at various stages in the cloning process and found the best results were achieved if activation occurred one to three hours after the genetic material was transferred.

Although the latest studies were done only in mice, they will still encourage scientists working on other mammals.