Free radicals, produced during metabolism, are molecules with an unpaired electron that may convert healthy cells to cancerous ones if too many are present.

"When you see something that looks different between normal cells and cancerous ones, you would like to think it's causally involved," but that remains to be established.

For years, Dr. Vogelstein said, researchers believed that control of the cell cycle was what divided normal cells from cancerous ones.

Dr. Malbon said he expected to be able to use MAP kinase to distinguish normal cells from cancerous ones.

The lung tissue was examined for the many changes that are believed to be involved in the progressive transformation of normal cells into cancerous ones.

Those false building blocks would inhibit the growth of unwanted cells, like cancerous ones, by fooling them into thinking that they were replacing themselves.

More recent research has revealed that genetic malfunction can be caused by chemical carcinogens that convert normal genes into cancerous ones.

This is because chemotherapy is toxic to healthy cells as well as cancerous ones.

Dr. Damadian thought that magnetic resonance could distinguish normal tissues from cancerous ones and, in 1971, published an article in the journal Science saying that.

Chemotherapy and radiation kill normal dividing cells as well as cancerous ones.