The other study, led by [Neil Caporaso], implicates one gene as playing a role, through other genes are probably also involved. The study focuses on a gene-known as CYP2D6-which causes cells to make an enzyme needed to break down and process a variety of substances that enter the body, including foods, drugs and various other chemicals.

The enzyme functions more efficiently in some people than others. Caporaso found that people in whom it functioned poorly or not at all-about 40 percent of those he studied-had a lung cancer rate one-sixth that of people in whom the enzyme worked at full efficiency. Caporaso speculates that the enzyme is needed to turn the chemicals in cigarette smoke into carcinogens. With little or no enzyme, the smoke is less harmful.

Caporaso said that this by no means proves that CYP2D6 is the lung cancer gene identified by [Thomas Sellers]' study. It is likely, Caporaso said, that more than one gene is needed. He said it is also possible that the key component could be another different gene that is simply inherited with CYP2D6.