When she started out two years ago at the University of Wisconsin-Madison searching for gene combinations that could reverse an ordinary skin cell into a stem cell, a primitive type that could potentially be turned into any kind of human cell, Yu Junying's chances were one in about nine followed by 157 zeros. 'I did not have much hope of getting anywhere when we started,' said Dr Yu, 34. But the Peking University graduate, who grew up on a rice farm in Tanxi village, Zhejiang province, and went to the US a decade ago, was undaunted and came up with a solution that could put her in the running for a Nobel Prize. 'I am just a stubborn person who got lucky,' she said of her breakthrough research. Stem cells became the source of intense scientific interest after researchers discovered they had the potential to produce any kind of human cell. The first human stem cells were successfully harvested in 1998 and since then many scientists have believed the only viable source of the type is an early-stage embryo. But the process involves destruction of the embryo and is ethically controversial. A number of alternative sources have since been proposed, including ova and sperm, but little progress has been made. Fresh from her PhD programme at the University of Pennsylvania, Dr Yu, started her research in 2003 as an assistant scientist in the laboratory of James Thomson, a renowned stem cell specialist who led the production of the first human embryonic stem cell lines. Dr Yu spent the next two years trying to prove that 'fusing' stem cells with other cells causes some of these cells to revert to a stem-cell-like undifferentiated state. 'It was an exciting finding, but practically useless unless we could figure out what were the driving genes behind the changes,' said Dr Yu. She calculated there were about 100 possible trigger genes in the pool and, in 2005, with the continued support of Dr Thomson, started sorting them out to determine which ones activated the process. She focused her efforts on setting off the reversal in skin cells. 'Few had thought about skin cells maybe because they are too marginal and too common,' said Dr Yu. 'But they are the type of cells we understand most.' About the same time, she had heard of a team of Japanese scientists who were on the same trail, but said she felt no pressure. 'They must have felt the same. In the beginning you feel as though only God knows how long it will take. It could be forever,' she said. Dr Yu worked at least 10 hours each day in the lab doing the labour-intensive cloning, injecting and testing of one gene after another, over and over again with only two technicians to help her. A year passed with no positive results. 'Besides the Japanese, I didn't think anyone else was willing to do the job because it was heavy duty and labour intensive,' she said. 'I was prepared for decades [of research].' But then she got lucky. On July 4 last year, Dr Yu hit on a combination of 14 genes that actually worked. To celebrate, she went home and had a sound sleep. The initial excitement was followed by fear and stress. For more than three years she had worked with little hope and, thus, little pressure. But at that moment she realised she could not 'afford to lose the race to the Japanese'. That realisation was shared by the university laboratory and more resources were put at her disposal, with Dr Thomson participating in the final testing phase. Dr Yu redoubled her efforts and successfully narrowed the number of genes down to four - two functional drivers and two efficiency boosters - and submitted her paper to Science magazine. The Japanese team, led by Kyoto University's Shinya Yamanaka, also submitted their findings - with two pairs of genes similar to Dr Yu's - to Cell magazine at more or less the same time. In a rare moment of co-operation between two rival scientific journals, the magazines decided to publish their papers simultaneously on November 20. 'It was a deal to share the Nobel prize,' a leading mainland biologist said. When asked recently if the paper would win her the Nobel Prize, Dr Yu simply said with a smile: 'Who knows?' Unlike the discovery of the molecular structure of DNA, which went unnoticed for years, Dr Yu's and Professor Yamanaka's findings made headlines around the world. 'Everyone was waiting for this day to come,' Tadeusz Pacholczyk, director of education at the National Catholic Bioethics Centre, told The New York Times. Pei Duanqing, deputy director general of the Chinese Academy of Sciences' Guangzhou Institute of Biomedicine and Health, said he vividly remembered a visit to meet Dr Yu at her lab last year. 'The woman has a very pure mindset ... with persistence and fresh ideas,' Professor Pei said. 'We are profoundly happy not only because it opens a door that leads to an entirely new world [of stem cell research] for all of us, but also because a Chinese has played a decisive role in making it happen.' Meanwhile, Dr Yu has two issues to address. First, she is eager to find out how and why the genes can effectively reverse a cell's development. 'We still know terribly little about the reversing mechanism. Further investigation may revolutionise our understanding of life,' she said. The second objective is to perfect the technology so that one day it can be safely applied to people and this involves finding ways to remove the four alien genes. 'The reversed stem cells are not exactly the same as those we obtain from an embryo ... [The alien genes] could induce mutations or cancer, so I am looking for methods to take them out after they complete their functions at the final stage,' she said. 'Before their removal, we will not use them to produce any tissues or organs for humans ... We may use [the stem cells] to study some diseases such as Parkinson's by creating some neurons for drug testing, but we are not ready to apply the cells on humans directly.'