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The Independent Culture
More than 4,000 illnesses, such as severe combined immunodeficiency (Scid) and cystic fibrosis, are caused by an inherited mutation (or fault) in just one gene. These are known as "single fault" genetic diseases.

Every other ailment - from cancer through heart disease and Aids to senility - is caused by a fault, either inherited or developed during our lifetime, in one or more genes involved in the body's defence mechanisms.

Gene therapy works by inserting a functioning gene into a cell to produce a beneficial protein or by preventing a defective gene from causing disease. In the first US government approved trial in 1990, a gene-producing adenosine deaminase (ADA), an enzyme vital for the proper functioning of the immune system, was in- serted into the cells of a four-year-old girl whose own ADA genes were not working. Since then over 1,000 patients have had genes inserted into their cells in 106 government-approved clinical trials in the USA. Around a third are for "single fault" genetic diseases. Most of the remainder are for cancers, which are caused by the mutation of several genes. The most common treatment here is to insert a gene producing a protein that induces the body's immune system to attack the cancer cells.

By the year 2005 the Human Genome Project hopes to have identified and located all the active genes in the human chromosome. But it is not enough to know which malfunctioning gene or genes cause which disease. A major problem hindering gene therapy is to find a safe method of delivering the corrective gene to enough of the right cells so that the gene produces the right amount of the desired protein over a sufficient period of time to be effective. The most promising delivery systems - called vectors - package the gene in a virus which has had its pathogenic elements removed.