The Supreme Court ruled 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 high court's unanimous judgment reverses three decades of patent awards by government officials. It throws out patents held by Salt Lake City-based Myriad Genetics Inc. on an increasingly popular breast cancer test brought into the public eye recently by actress Angelina Jolie's revelation 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," Thomas said.

Patents are the legal protection that gives inventors the right to prevent others from making, using or selling a novel device, process or application. The U.S. Patent and Trademark Office has been awarding patents on human genes for almost 30 years, but opponents of Myriad Genetics Inc.'s patents on the two genes linked to increased risk of breast and ovarian cancer say such protection should not be given to something that can be found inside the human body.