NEW YORK (AP) -- Scientists say they've found a big reason why treatment for chronic hepatitis C infection works better for white patients than for African-Americans. It's a tiny variation in a gene.

People with a certain gene variant are far more likely to respond to treatment, and that variant is more common in people with European ancestry than African-Americans, researchers report.

In fact, that probably explains about half the racial disparity in treatment response, the scientists estimate in a study published online Sunday by the journal Nature.

The work involved 1,137 patients who had a chronic infection with the most common type of hepatitis C virus found in the United States and Europe, one that is is less responsive to treatment than other types. They were given standard drug treatment.

Analysis showed the treatment wiped out the virus in about 80 percent of study participants with the favorable genetic variant, compared to only about 30 percent among those who lacked it.

African-Americans who had the gene variant showed a better response rate than whites who didn't have it, indicating that the gene is a better predictor than ethnicity, the researchers said.

Standard treatment for chronic hepatitis C takes months and has such potential side effects as flu-like symptoms and depression. So in deciding whether to treat, patients and doctors might be helped by testing for the variant first, say the study authors, from Duke University and elsewhere.

An estimated 170 million people around the world and perhaps 3.2 million people in the United States alone have a chronic hepatitis C infection. That puts them at risk for developing liver cirrhosis and cancer. The virus is spread by contact with infected blood, such as from using inadequately sterilized needles.

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