The drug company's vice-president and worldwide director of genetics, Dr Allen Roses, told a conference in San Francisco that it had achieved this by using a gene mapping technology to track down disease-causing genes.

Dr Roses said the technology, known as Single Nucleotide Polymorphisms (SNP), could now narrow the search for susceptibility genes to a small region containing only one to eight genes within a relatively large region of DNA known to be linked to a particular disease.

This would help scientists to discover more rapidly which genes cause people to develop diseases.