Gene variant linked to early puberty. (Biomedicine: from New Orleans, at a meeting of the American Association for Cancer Research).

Girls with a version of a gene that boosts their body's ability to break down testosterone reach puberty earlier than girls whose bodies metabolize this male sex hormone more slowly. Earlier puberty is a known risk factor for breast cancer.

Most scientists blame childhood obesity for the hastening of puberty. But genes may also play a role, report Fred F. Kadlubar of the Food and Drug Administration's National Center for Toxicological Research in Jefferson, Ark.

Among 192 girls, 90 percent of those with two copies of the gene variant for faster testosterone metabolism developed breasts by the time they were 9.5 years old. Only 40 percent of girls with a gene variant corresponding to a slower breakdown of testosterone had developed breasts by this age.

"Surprisingly, none of the genes associated with estrogen metabolism seemed to have anything to do with early puberty," Kadlubar says. Drugs that interfere with testosterone metabolism aren't a treatment option. A key enzyme they affect is also important to other metabolic processes, he explains.

One safe way for girls to reduce the risk of early puberty is to stay at a healthy weight and be physically active, comments Christine Ambrosone of the Mount Sinai School of Medicine in New York.--D.C.