But recently, patients suffering from Huntington's disease and other genetic diseases were given hope by Professor Beverly Davidson at the University of Iowa. It takes the form of a novel method, already successful in animals, exploiting a fundamental mechanism of all life, which goes by the forbidding title of RNA interference (RNAi). Also known as 'gene silencing', RNAi is being greeted as the next quantum leap in medicine by scientists the world over.

For once the prospects match the hype. Sir Paul Nurse, Britain's whiz-kid Nobel-prize winner, is this country's leading RNAi enthusiast. The head of Cancer Research UK, who is soon to depart for New York to run the Rockefeller University, where he will have 22 Nobel winners as colleagues, Nurse, 54, sees the technique as a 'fantastic new tool' for discovering how each of the 35,000 human genes contribute to cancer. 'Thanks to the incredible discovery of RNA interference, we think we should be able to crack the problem.'

RNAi is hailed as the greatest breakthrough of the year by the influential American journal Science, which announces in a lead editorial that it has split the field of medical research 'wide open', putting the new discovery at centre stage: 'Having exposed RNA's hidden talents, scientists now hope to put them to work.' Its potential, according to researchers, is astonishing. In plant and animal models, RNAi techniques have resulted in making fat organisms thinner, bigger organisms smaller, and even promoting longevity. Researchers at Cancer Research UK, in collaboration with a research centre in Amsterdam, are now in a race with America's top genetic research institute, Cold Spring Harbor Laboratory on Long Island, putting the method to work to explore in depth the mechanisms of every gene that could contribute to cancer.

So what is RNAi? Our genes are the codes that instruct our cells to make specific proteins, the building blocks of blood, tissues, nerves and so forth of the complete body. Those same proteins can sometimes be the building blocks of genetically based diseases that plague the human race, including cancers and conditions such as Huntington's. These proteins also control whether we will tend to be fat or thin, prone to alcoholism or drug addiction, energetic or lazy; even if we will be long- or short-lived. Until recently RNA (which stands for ribonucleic acid) was considered to be merely a kind of message system that transmitted the information coded in our genes to the workshops of the protein-building areas of our cells.

Scientists are now conceding that they have tended to neglect the full significance of the complex and central importance of these messengers. What researchers have been attempting in recent decades is to cure genetically based illnesses by inserting new genes, or even attempting to knock out particular genes entirely. This strategy has proved difficult and hazardous. But by directing their focus instead onto the RNA message system, they believe they can more easily and less dangerously slow down or completely switch off the production of harmful proteins in the human body.

We can understand the operation of the DNA of our genes, RNA and proteins by comparing them to the creation of a large orchestral piece of music, like a symphony. The composer writes out a master score of the symphony. Individual copies of each orchestral part are then made and distributed to the performers, who then make the notations a vibrant musical reality. If the composer's original master score is the gene, and the actual performances, the sounds made by each of the instruments - violins, violas, flutes, trumpets, drums - the proteins, then the RNA messengers are the instrumental scores, made from the master score, for each of the performers.

The principle involved in RNAi is not to attempt to meddle with the master score, the gene, but to interfere, rewrite, or even delete, parts of the individual instrumental scores, thus 'silencing' specific performances, or parts of a performance in a highly targeted fashion. If the composer has made mistakes in his master score, which translate into unpleasant dissonances in the actual music, RNAi is the means of tackling those mistakes by addressing the individual parts of the orchestra to 'silence' the instruments that create the problem.

Another way of understanding RNA is to compare the development of an organism, whether it be a plant, a worm, a fly, a human body, with the making of, say, a complex seagoing vessel. If the DNA is the ship designer's impression of a complete ship, the RNA consists of the essential detailed, three-dimensional drawings for the shipwrights to follow. RNA interference does not attempt to change the principal design: it goes for the detailed drawings where it can target highly specific features before they are translated into actual joists, planks, masts, rails and rudder. In technical or biological terms, a gene expresses itself, or begins the process whereby it makes proteins, by unzipping the two strands of the DNA double-helix genetic code so as to allow one of the complementary strands to be copied as a strand of the molecule RNA. At this stage the copied strand is known as messenger or mRNA. The mRNA passes from the nucleus of the cell into the cytoplasm (the workshop of the cell), where it acts as a template for the synthesis of proteins. Researchers have found that diced-up shorter sections of the RNA molecule can interfere with the protein-building operation, toning down or switching off the protein-building processes. Now researchers are beginning to introduce these short interfering RNA strands artificially into the cells of plants, fruit flies, mammals and human cells, thereby discovering the subtle mechanisms that control protein-building.

The diseases that stand to benefit from RNAi therapy include Alzheimer's, breast cancer, leukaemia, schizophrenia and many, many more. But it looks as if Huntington's disease will be one of the illnesses tackled earliest by the RNAi process. Leonore Wexler's tragic case history, with which we began our story (she died 30 years ago), became famous in the annals of medicine because Nancy Wexler, one of her two daughters, made a search for HD her life's work. Today she is the president of the Hereditary Disease Foundation in New York and a professor at Columbia University medical school. Nancy Wexler, who herself has a 50-50 chance of contracting the disease, began studying a community on the shores of Lake Maracaibo, Venezuela, where there is an unusual incidence of Huntington's disease. She returned year after year, assiduously collecting DNA samples and carefully tracing family histories.

Finally, she joined forces with a researcher from the Massachusetts Institute of Technology (MIT), James Gusella, who was studying an Iowa family with HD. In 1983, Gusella identified the HD gene. But it was to take 150 researchers another 10 years to isolate the gene for detailed analysis. Eventually they found that the disease is caused by mutations in the Huntington's gene that generate a protein that destroys parts of the brain. To attempt to knock out the gene entirely would deprive the patient of other important proteins necessary to life. But RNAi works to isolate and reduce or switch off the precise processes that build the damaging proteins, while leaving the gene intact. 'When I heard of this work,' said Nancy Wexler recently, 'it just took my breath away.'

Optimism for RNAi research, which could one day lead to highly targeted drugs rather than complicated genetic 'carrier' systems, contrasts sharply with the widespread failure of genetic therapies. One tragic victim of genetic therapy was Jesse Gelsinger, an American teenager from Arizona who was suffering from ornithine transcarbamylase deficiency (OTC), a hereditary disease of the liver that prevents the patient from producing urea. In 1999, when Jesse was 18, he volunteered to undergo gene treatment even though the illness was not life-threatening. The therapy was conducted by James Wilson, the director of the genetic therapy institute at the University of Pennsylvania. The strategy involved the administration of a virus that carried a correcting gene into the patient's cells. A few hours after the treatment, Jesse developed a fever followed by an uncontrollable infection with blood clots and haemorrhage. Three days later he died. The therapy killed the patient.

There have been two other gene-therapy deaths, involving two French children suffering from a genetic illness known as SCID, an immunodeficiency illness made famous in a television drama called The Boy in the Plastic Bubble. In 2000, a team in Paris performed gene therapy on the children who had been kept in sterile isolation since birth. Employing a method for genetically spreading immune cells into the bodies of the children, the therapy seemed to work at first. But in October 2002, one of the children developed leukaemia. James Watson, who won the Nobel prize for joint discovery of DNA, has remarked that 'though it has not been established for sure that the genetic procedure was responsible, the circumstantial evidence is mighty strong'. Watson adds that 'gene therapy seems to have cured the baby's SCID, but caused leukaemia as a side effect'. In February of this year the other child was also diagnosed with leukaemia.

Explaining the biology of RNAi involves a rehearsal not only of the story of discovery of the structure of DNA, but of the history of life itself from its first origins on Earth. In March 1953, after a frenetic round of research, detective work, and excited talk over pints of beer in Cambridge pubs, Francis Crick, a former wartime engineer and physicist, and James Watson, his young American biologist colleague, produced their famous model of DNA made from wire and cardboard. For the first time the world understood the shape and operation of the double-helix structure of the genetic code of inheritance whereby organisms are reproduced and life is passed from generation to generation.

DNA, according to Watson and Crick, was a kind of mother of all molecules, which was to become the principal focus of basic research and quests for treatments for everything from cancer to Huntington's disease.RNA molecules, however, were seen as no more than drones, taking orders from DNA, to convert genetic information into the body's essential proteins. But is it possible that the pioneers of molecular biology overemphasised the importance of DNA, and underestimated the role of RNA? What if the terms in which they stated the central importance of DNA hampered other ways of looking at the secrets of life? That is the conclusion of a growing number of researchers who are focusing on the crucial importance of RNA, DNA's close molecular relative.

Biologists who study the origins of life are now speculating that it was RNA all along that was the earliest form of encoded life rather than DNA. That RNA was considered the poor relation of DNA owes much to the way in which scientists make metaphors in describing the science of nature. When Francis Crick presented his DNA discovery he announced what he called its 'central dogma', using language borrowed from information theory, popular at a time when computer science was rapidly expanding. DNA, the double-stranded molecule, contains, as we know, the coded instructions for the protein building blocks of an organism. A copy of these genetic instructions, as we have seen, is made as the single-stranded molecule RNA. Crick called this copying process 'transcription'. The RNA strand now moves out into the cytoplasm of the cell, where the proteins are formed. Continuing the information-theory parlance, this final process Crick called 'translation'. According to Crick, this was a one-way path. DNA goes to RNA, which creates proteins.

RNA, by this model, was seen as a mere one-way messenger, a mere transmitter of information.A token of the power of that latent informational image can be seen in Richard Dawkins's bestselling book about evolution, The Blind Watchmaker. He is looking in wonder at a willow tree in seed outside his window. 'It is raining DNA,' he writes. 'It is raining instructions out there; it's raining tree-growing, fluff-spreading algorithms. This is not a metaphor, it is the plain truth. It couldn't be any plainer if it were raining floppy discs.'

One problem with Dawkins's powerful image, which is essentially true, is the absence in the picture of RNA. The role of RNA as mere messenger in the process of life is now being reassessed far and wide by scientists, pharmaceutical companies, and people who invest in high-tech medicine. Ironically, shortly after the Crick and Watson discovery, Crick raised questions about why the information in DNA had to be mediated by RNA before a protein could be made. Crick consequently suggested that RNA actually predated DNA, speculating that there must have been a time when all life existed in an 'RNA world'. In 1954, as if to lay claim to RNA's past and future bid for primacy, Crick and his colleagues, in typical larky researcher style, founded the RNA Tie Club, restricted to 20 members, corresponding to the number of amino acids encoded by RNA. The members would sport a tie and matching tiepin made by a haberdasher in Los Angeles, with diagrams illustrating each of those amino acids.

RNA is essentially dynamic and unstable, according to the earlier view of the molecule, which is why it gave precedence to the stability of DNA as the best long-term storage system. It now appears that Sars is an RNA-based organism rather than a DNA-based one. But it was not until 1983, 30 years after the DNA discovery, that Sidney Altman at the University of Colorado demonstrated that RNA molecules were not mere messengers but could catalyse their own self-replication. DNA could not exist, affect organisms or reproduce without RNA; but RNA on its own could support an 'RNA world', as biologists like to call it. Altman won the Nobel prize for chemistry in 1989 for his discovery.

James Watson, Crick's collaborator, celebrating the 50th anniversary of the discovery of DNA, recently commented on the profound importance of RNA. Which came first, asked the veteran biologist: proteins, which have no known means of duplication, or DNA, which can duplicate information but only in the presence of proteins? 'The problem is insoluble,' he declared. 'You cannot have DNA without proteins, and you cannot have proteins without DNA.'

But Watson now answered his own question: 'RNA, being a DNA equivalent, can store and replicate genetic information. It is also a protein equivalent as it can catalyse critical chemical reactions. In fact, in the 'RNA world' the chicken-and-egg problem simply disappears. RNA is both the chicken and the egg.' Watson is talking of the early stages of life on the planet, distant evolutionary circumstances billions of years ago. But the emerging importance of RNA for today is not only its closeness to DNA and to proteins, but its amazing complexity, its ability to operate in many different formats and lengths as message switch systems - modulating the expression of proteins, turning them on and off, and shaping genes themselves.

A scientist involved with important research and therapy strategies for cancer in the UK and for Europe puts the protein phenomenon in context. 'It's all very well knowing that a particular gene can cause cancer,' he told me at the site of a new £140m oncology centre at Cambridge, 'but the real problem is identifying which proteins are expressed by that gene so as to create the cells, tissues, blood that form tumours or leukaemias.' The scientist is Professor Bruce Ponder, the head of what will soon become Europe's largest cancer research centre, with up to 500 scientists. Ponder was associated with the discovery of the breast-cancer genes BRCA 1 and BRCA 2, and he is well placed to pinpoint the next stages in the battle to cure cancer.

Identifying the genes that cause cancer and myriad other diseases, by attempting to manipulate or eliminate the DNA in the cell's nucleus, has not, as we have seen, translated into a successful treatment strategy. The vector viruses used in these therapies appear to interfere with the patients' DNA in such a way as to cause cancer.

Lack of progress has cast a shadow over the high expectations for the eventual rewards that were to flow from the discovery of the structure of DNA in Cambridge 50 years ago. But what if scientists could now find ways of identifying all those damaging proteins that Professor Ponder refers to? And what if they could find methods of accurately switching off the protein products that cause damage or abnormalities without affecting the body's DNA? And what if such 'abnormalities' could extend beyond cancer to problems like obesity, the effects of ageing and the estimated 4,000 diseases associated with our genes? That, in the view of researchers, now seems a closer goal as a result of RNAi discoveries.

What has prompted this paradigm shift in gene-expression research 50 years on from the DNA discovery? The breakthrough began in 1990 with a humble horticultural experiment, when a group of plant biologists tried to create a petunia with a deeper shade of pink flower. They had introduced strands of RNA into the cells of a petunia in the belief that they could intensify the protein effects of a gene associated with colour. To their astonishment, the petunia sprang not purple flowers but pure white ones. They then realised that RNA was capable not only of increasing the effect of a protein but of switching it off or turning it down. Next, a group of scientists experimenting with nematode worms found that they could alter the metabolic rate of the creatures, making them fatter or thinner, even coaxing them to live longer, by tweaking their RNA with RNA interference. Soon researchers were working with fruit flies, then mice, and then with human cells in Petri dishes to control the building blocks of proteins by interfering with their RNA mechanisms. There were problems and false starts in plenty, but the word was out that RNAi beckoned a crucial new era in genetic research.

In the laboratory complex of Cancer Research UK in Lincoln's Inn Fields, London, Professor Julian Downward is running an RNAi research programme in collaboration with a Dutch group. Downward, 42, dressed in a dark-blue golf shirt, sits back languidly in his office chair, talking RNAi. He is one of those laid-back scientists who tend to avoid hype as if it were a death-dealing virus. But he cannot hide his enthusiasm for his work. 'In times gone by,' he says, 'it took a few years before the very latest technologies filtered down to charities and academic laboratories, so it's really exciting to be among the pioneers of the use of RNA interference for cancer research.'

Downward and his Dutch collaborators are building a 'library' of the link between genes and proteins, using RNAi as a research tool. He is seeking to establish what researchers need to take away or switch off in a cancerous cell to make it normal again. No ordinary library, this: it exists in the form of trays of cells containing engineered viruses expressing RNAi for each of the human genome's 35,000 genes. It is a long, painstaking process, requiring day-and-night automative methods. So far, Downward and his colleagues have processed 8,000 human genes.

The information gleaned from the 'library' will reveal the myriad protein products of human genes, as well as showing how RNAi can knock out the expression of genes in health and sickness. The knowledge will prove a great boon to pharmaceutical companies working on designer drugs for cancer aimed at tackling the harmful proteins direct, as well as paving the way for high-tech RNAi therapies aimed at knocking out harmful proteins by addressing the cells' RNA.

Pharmaceutical companies are pumping hundreds of millions of dollars into RNAi research and huge excitement is spreading through the industry, especially in the US. Sirna Therapeutics, for example, a Colorado-based biotech company that has taken out a clutch of 180 patents relating to nucleic acid (the basis of all RNA), has redirected its research into RNAi. A recent corporate announcement claimed that RNAi-based drugs could become the therapeutics of the future. Last year, a new biotech company in Boston, Alnylam Pharmaceuticals, raised $17m to fund RNAi research under the auspices of the Nobel-prize winner Philip Sharp. In San Diego, Anadys Pharmaceuticals raised $38m for RNAi research. 'By assembling the right set of tools to systematically go after RNA, we're opening up the other half of the playing field,' says Anadys's director for investor relations.

A new boost for the drive against cancer is clearly fuelling the RNAi fever; but there are other benefits beckoning the investors. As yet, the circumstances are wrapped in the technicalities of science, but a leading article in Nature earlier this year revealed more than a hint of possibilities across the horizon. Discussing a 'genome-wide'

RNAi analysis of 'fat-regulatory' genes in a worm known as caenorhabditis, scientists based at the Wellcome/Cancer Research UK Institute in Cambridge claimed that they had identified a 'core set of fat-regulatory genes as well as pathway-specific fat regulators'. The scientists declared that these are genes shared with mammals, and concluded that this could 'point to ancient and universal features of fat-storage regulation, and identify targets for treating obesity and its associated diseases'.

Professor Tom Kirkwood, who delivered the Reith lectures four years ago and is a world expert on ageing, talked to me about the effect of the new research on longevity. 'We've known for some time that there are several genes whose mutation can extend life span in nematode worms,' he said. 'RNAi is a neat way to carry out these kind of studies that have previously been done by gene knockouts.'

By studying the ageing process in model organisms like worms and flies, he told me, 'we are sure to learn some useful things about how ageing occurs in people'. He is cautious, however, about translating this into a quick way of delivering longer life to human beings. 'For starters,' he said, 'the body of an adult worm has no dividing cells, whereas we have many organs and tissues whose repairs and renewal depends heavily on continued cell replacement.'

In the meantime, scientists such as Nancy Wexler would be happy to see a breakthrough in the disease that threatens her own life and that killed her mother. If RNAi can prove successful in just one genetic disease, it will offer hope for sufferers in many others. Sir Paul Nurse, as he prepares to go to the Rockefeller University, has his eyes and his own continuing research set steadily on cures for cancer, but he does not blench at dropping hints about wider comparisons between animal research and human applications. 'Through RNAi research we are discovering what gives animals big brains, or makes them fat, or makes them stupid, and making adjustments. It is more difficult, of course, but there is no reason why we will not one day do the same thing in humans.'