A Tokyo geneticist said Wednesday his research team has created the world's first fourth-generation cloned pigs, an achievement that could help scientists in medical and other research.

The male pigs were born at Meiji University in July, said Hiroshi Nagashima, the geneticist at the university who led the project.

Earlier attempts to clone animals for several generations were problematic, and scientists had thought this was because the genetic material in the nucleus of the donor cell degraded with each successive generation, Nagashima said.

But the team's findings show that a large mammal can be cloned for multiple generations — in this case, the clone of a clone of a clone of a clone — without degradation, he said, while acknowledging that mice have already been successively cloned for multiple generations.

Akira Onishi, a geneticist with the government-affiliated Agriculture, Forestry and Fisheries Research Council, said Nagashima's animals could be the world's first fourth-generation cloned pigs, an achievement that could help other cloning researchers.

"I am not aware of any other cases," said Onishi, a member of the team that produced Japan's first cloned pig.