The discovery - which reveals that the rogue gene is inherited from the mother - is the result of a 10-year international drive to uncover the secrets of a form of cancer which is becoming increasingly common.

This should allow the disease to be detected earlier, and be treated successfully with less aggressive chemotherapy

Dr Doug Easton, The Cancer Research Campaign

Three UK cancer charities have collaborated on the project: the Cancer Research Campaign, the Imperial Cancer Research Fund and the Institute of Cancer Research.

The researchers have discovered that the vital gene is located on a small part of the X chromosome, which is inherited from the mother.

This means that they are just one step away from identifying the gene itself.

They believe the discovery will help them in the treatment of the disease and other forms of cancer.

Testicular cancer is the most common cancer in men aged 15-40 in the UK, affecting more than 1,400 people every year.

The number of men in England and Wales who develop the disease increased by more than 80% between the early 1970s and early 1990s.

The newly located gene, which has been called TGCT1, makes men who carry it more susceptible to testicular germ cell tumours (TGCT) which make up 95% of all testicular cancer cases.

Many cases are genetic

Testicular cancer responds well to chemotherapy

Institute of Cancer Research scientist Professor Mike Stratton said: "TGCT1 looks to be the first of an unknown number of genes � almost certainly more than three - which predispose men to the disease.

"At the moment we simply do not know what percentage of testicular cancer cases are caused by an inherited genetic susceptibility, but some estimates have put it as high as 20% of all cases."

Dr Doug Easton, of the Cancer Research Campaign's Genetic Epidemiology Unit, Cambridge University, said: "Once the gene has been identified, we will be able to distinguish men at high risk of the disease, who can then be screened.

"This should allow the disease to be detected earlier, and be treated successfully with less aggressive chemotherapy."

It should also lead to a greater understanding of the biology and genetics of testicular cancer itself, which could help in the treatment of other cancers.

Professor Stratton said testicular cancer was particularly susceptible to chemotherapy, which means it is successfully treated in more than 90% of cases.

"If we can find out why this is, we might be able to make other cancers equally sensitive."

Researcher Professor Timothy Bishop, of the Imperial Cancer Research Fund Genetic Epidemiology Lab in Leeds, said: "'We have also found that TGCT1 is more likely to be involved in families in which a man had an undescended testicle - which we already know is one of the risk factors for TGCT1.

"What's more, we discovered that families of men who had cancer in both testicles were even more likely to have TGCT1."

Volunteers needed

The UK scientific team worked with colleagues in Norway, Germany, Australia, Ireland and Canada to study a total of 134 families with two or more cases of testicular cancer.

Out of these families, 87 included two brothers who both had testicular cancer.

Members of the research team will now continue their work to isolate TGCT1, which they believe may take another two to three years.

They are also looking to locate other inherited testicular cancer genes.

To help them in their work, they are urgently appealing for volunteers with two or more cases of testicular cancer in their family to come forward.

Anyone who can help should write to Dr Robert Huddart, Academic Unit of Radiotherapy and Oncology, The Royal Marsden NHS Trust, Downs Road, Sutton, SM2 5PT.