Toronto, July 16 - New research by Canadian scientists is set to turn upside down the DNA theory which says that every cell in the body is identical to every other cell.

Current genetic studies to know the causes of many diseases involve blood samples from the patients on the presumption that all cells in the body are same.

But a new study by researchers at Montreal's McGill University has found differences in cells from blood and body tissue.

Researchers also say that except for cancer, samples of diseased tissue are difficult or even impossible to take from living patients.

The vast majority of genetic samples used in large-scale studies today come in the form of blood, but the researchers have found that blood and tissue cells do not match genetically.

Thus, the current expensive genomic studies may prove to have been essentially flawed from the outset, according to the new study.

The discovery came to light when the researchers were studying the genetic causes of abdominal aortic aneurysms (AAA), which is one of the rare vascular diseases where tissue samples are removed as part of patient therapy.

But when the researchers compared the tissue samples, they found major differences between BAK genes in blood cells and tissue cells coming from the same individuals, with the suspected disease 'trigger' residing only in the tissue. BAK genes control death cells.

The same differences were later found in samples from healthy individuals, a university statement said Wednesday.

'In multi-factorial diseases other than cancer, usually we can only look at the blood,' said Bruce Gottlieb, a geneticist at McGill's University's Centre for Translational Research in Cancer.