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.