When Genae Girard received a diagnosis of breast cancer in 2006, she knew she would be facing medical challenges and high expenses. But she did not expect to run into patent problems.

Ms. Girard took a genetic test to see if her genes also put her at increased risk for ovarian cancer, which might require the removal of her ovaries. The test came back positive, so she wanted a second opinion from another test. But there can be no second opinion. A decision by the government more than 10 years ago allowed a single company, Myriad Genetics, to own the patent on two genes that are closely associated with increased risk for breast cancer and ovarian cancer, and on the testing that measures that risk.

On Tuesday, Ms. Girard, 39, who lives in the Austin, Tex., area, filed a lawsuit against Myriad and the Patent Office, challenging the decision to grant a patent on a gene to Myriad and companies like it. She was joined by four other cancer patients, by professional organizations of pathologists with more than 100,000 members and by several individual pathologists and genetic researchers.

The lawsuit, believed to be the first of its kind, was organized by the American Civil Liberties Union and filed in federal court in New York. It blends patent law, medical science, breast cancer activism and an unusual civil liberties argument in ways that could make it a landmark case.