Six cloned calves have been created from cells kept growing in a test-tube for up to three months.

The breakthrough shows that the cells used for cloning do not have to be 'fresh'. This increases the chances that endangered species could be safeguarded by storing their tissue. It will also allow further investigation of whether clones age prematurely.

Dr Jerry Yang, head of the University of Connecticut's Transgenic Animal Facility, led the research and said the team was surprised to find that stored donor cells were able to be successfully cloned.

Storage better

The storage may even be beneficial, according to Dr Yang: "We observed higher developmental rates for embryos derived from donor cells after long-term culture than those after short-term culture."

The experiments were done with the Kogashima Cattle Breeding Development Institute in Japan. Skin was taken from the ear of a prize Japanese Black bull called Kamitakafuku.

The 18-year-old pedigree bull is famous in Japan for the superior meat of the calves it sires. It has already fathered nearly 160,000 offspring.

Cells from the skin were used to create six genetically-identical calves by cloning. Four of the cloned bulls are still alive but two have died, one at birth and the other from an infection.

The eldest calves are now over one year old, but their existence was only revealed recently.

"This is very powerful stuff," said Dr Thomas Wagner, at Clemson University. "The approach is infinitely more valuable scientifically and commercially than that which has been done before."

The full results will be published in the journal, the Proceedings of the National Academy of Sciences.