Los Angeles ? As many as 20,000 American women with breast cancer could be spared the side effects and cost associated with the drug paclitaxel because their tumors do not respond to it, according to the results of a genetic study reported Thursday.

Using new techniques to re-analyze a 1994 clinical trial, the team found that only women whose tumors produced a protein called HER-2 responded to the drug. Those women had a 40 percent reduction in the recurrence of their cancer, whereas women who had HER-2-negative tumors received no benefit, the researchers reported in the New England Journal of Medicine.

“The days of ‘one size fits all’ therapy for patients with breast cancer are coming to an end,” Dr. Anne Moore of Weill Cornell Medical College in New York wrote in an editorial accompanying the study.