Professor French Anderson, director of the gene therapy laboratories at the University of Southern California, Los Angeles, said the procedure had already been tested on laboratory animals with 'spectacular results'.
In December 1992, the first of eight human patients was treated in the United States. The American medical authorities and the patients' doctors 'are encouraged' by their progress.
But Professor Anderson warned: 'It is imperative that we go into the era of gene therapy as responsibly as possible and the responsible way is to use gene therapy for the treatment only of serious diseases.'
Human gene therapy began in September 1990, when the professor and colleagues treated a four-year-old girl for an extremely rare form of inherited immune deficiency. So far only 60 patients have been treated.
In the United States there are at least 20,000 patients each year suffering from glioblastoma multiforme, the type of brain cancer for which the eight patients are now being treated. It is the most malignant type of brain tumour and most patients die within a year of diagnosis. All eight patients undergoing the experimental therapy had been treated with chemotherapy or radiation without success.
Researchers inject genetically engineered cells taken from mice into the human patients' brains near the tumour. The mice cells are programmed to produce a modified herpes virus, which in turn infects the tumour cells. The treatment is selective because the virus will infect only cells that are dividing to produce more cells and, within the brain, the only dividing cells are those of the growing tumour and its blood supply.
Once inside the tumour cells, a component of the herpes virus insinuates itself into the human DNA. A week after the genetic treatment, the doctors give their patients gancyclovir, an anti-herpes drug, which the herpes gene then turns into a poison to kill its host tumour cell.
Dr Greg Winter, from the laboratory of molecular biology at Cambridge, told the congress that he could produce highly specific antibodies (one of the key components in the immune system) by genetically engineering a bacteriophage - a type of virus which preys upon bacteria.
By fusing fragments of antibodies on to the virus's outer coating, it is possible to devise a means of picking specific human proteins out of human blood. The approach could provide a 'magic bullet' to pick out proteins produced by cancer cells and deliver a tumour-killing chemical direct.
A gay geneticist, Dr Douglas Futuyma, called on the scientific community to set up a 'union of concerned geneticists' to prevent abuses of modern genetics, after last month's alleged discovery of a 'gay gene'.