The ruling throws out patents held by Myriad Genetics on a breast cancer test brought into the public eye recently by actor Angelina Jolie

The U.S. Supreme Court ruled on Thursday that companies cannot patent parts of naturally-occurring human genes, a decision with the potential to profoundly affect the emerging and lucrative medical and biotechnology industries.

The court’s unanimous judgment reverses three decades of patent awards by government officials. It throws out patents held by Myriad Genetics Inc. on an increasingly popular breast cancer test brought into the public eye recently by actress Angelina Jolie’s disclosure that she had a double mastectomy because of one of the genes involved in this case.

Justice Clarence Thomas, who wrote the court’s decision, said that Myriad’s assertion that the DNA it isolated from the body for its proprietary breast and ovarian cancer tests were patentable had to be dismissed because it violates patent rules. The court has said that laws of nature, natural phenomena and abstract ideas are not patentable. “We hold that a naturally occurring DNA segment is a product of nature and not patent eligible merely because it has been isolated,” said Mr. Justice Thomas.

The company has used its patent to come up with its BRACAnalysis test, which looks for mutations on the breast cancer predisposition gene, or BRCA. Those mutations are associated with much greater risks of breast and ovarian cancer. Myriad sells the only BRCA gene test. Opponents of its patents say the company can use the patents to keep other researchers from working with the BRCA gene to develop other tests.

Ms. Jolie revealed last month that she carries a defective BRCA1 gene and her doctor said the test that turned up the faulty gene link led Ms. Jolie to have both of her healthy breasts removed.

Companies have billions of dollars of investment and years of research on the line in this case. Their advocates argue that without the ability to recoup their investment through the profits that patents bring, breakthrough scientific discoveries to combat all kinds of medical maladies wouldn’t happen.

The court did rule that synthetically created DNA, known as cDNA, can be patented “because it is not naturally occurring”, said Mr. Justice Thomas.