The breakthrough was published in Developmental Cell, a leading international biology journal, on Monday. The article is also scheduled for publication in the top academic journal Science on Friday.

Researchers at Fudan's institute of developmental biology and molecular medicine said yesterday that the discovery came after they began studying the movement of the so-called SUN1 protein in the nuclear membrane of mice cells in 2003. When they deliberately broke the protein, the disruption prevented the normal pairing and recombination of chromosomes during the reproduction process in sexual cells.

Mice born with broken SUN1 protein had much smaller testes or ovaries than normal mice and did not produce sperm or eggs, researchers said.

"We can infer that the SUN1 protein plays a key bridging role in the reproductive process of mice, which also provides a target point for the diagnosis of human infertility," said Ding Xu, a PhD researcher at the institute and the article's lead writer.

Scientists have long understood the pairing and recombination process of chromosomes, though the initiating factor has remained unclear.

"A genetic reason for most infertility has been ambiguous for a long time," said Xu Ren'er, an associate professor and co-author of the article.

Ten percent to 15 percent of the world's couples suffer infertility problems, and the figure is about the same in China.

Worsening environmental pollution along with other factors have made infertility a growing concern, researchers said.

As mice and humans share about 99 percent of the genes that influence reproduction, "the finding is expected to provide a new clue for human beings to understand the reason for the disorder," Xu said.

He cautioned, however, that it's still too early to say how the finding might be applied to the treatment of human infertility.

The achievement was hailed by other scientists as well.

"The reviewer is very enthusiastic about this finding," the Developmental Cell's review panel wrote, noting that the Shanghai team was responsible for the world's first such discovery in mammals.