The Supreme Court ruled unanimously Thursday that human genes cannot be patented, a decision that is likely to shape the future of medical and biotech research.

Those who challenged the notion that something that occurs naturally in the human body can be “owned” by a company said the decision could open a new era of expanded research and lower costs for genetic testing.

At the same time, the ruling held out hope for industry that certain research methods and the creation of synthetic DNA can be protected and thus worth the investment.

“Today, the court struck down a major barrier to patient care and medical innovation,” said Sandra Park of the American Civil Liberties Union, which represented doctors, researchers and cancer patients who brought the challenge.

“Because of this ruling, patients will have greater access to genetic testing, and scientists can engage in research on these genes without fear of being sued.”

The issue received national attention last month when actress Angelina Jolie revealed that she had a double mastectomy because genetic testing showed she carried a defective gene that greatly increased her chance for cancer.

The ruling was a split decision for Myriad Genetics Inc., which holds patents on genes that have been linked to breast and ovarian cancer and thus is the only company that offers the genetic tests, which can cost more than $3,000.

Justice Clarence Thomas, writing for the court, said that merely isolating those specific genes — BRCA1 and BRCA2 — was not worthy of a patent. The decision was a departure from decades of decisions to the contrary from the U.S. Patent and Trademark Office.

“Myriad did not create anything,” Thomas wrote. “To be sure, it found an important and useful gene, but separating that gene from its surrounding genetic material is not an act of invention.”

He added that “groundbreaking, innovative, or even brilliant discovery does not by itself” overcome the fact that “laws of nature, natural phenomena, and abstract ideas” are beyond the domain of patent protection.

On the other hand, Thomas wrote, when Myriad and others create a synthetic form of DNA — called cDNA and particularly useful in conducting experiments and tests, such as cancer screening — their work does deserve patent protection.

“The lab technician unquestionably creates something new when cDNA is made,” he wrote.

Myriad focused on that part of the decision.

“We believe the Court appropriately upheld our claims on cDNA, and underscored the patent eligibility of our method claims, ensuring strong intellectual property protection for our BRACAnalysis test moving forward,” Peter D. Meldrum, the company’s president and chief executive, said in a statement. “More than 250,000 patients rely upon our BRACAnalysis test annually, and we remain focused on saving and improving peoples’ lives and lowering overall healthcare costs.”

The average American woman has a 12 to 13 percent chance of developing breast cancer, according to the court’s opinion, but mutations in the two genes isolated by Myriad increase the risk to as much as 80 percent for breast cancer and 50 percent for ovarian cancer. Myriad developed tests useful in detecting those mutations.

The court’s opinion required the nine justices to confront not only the mysteries of DNA — their struggle was apparent during the case’s complicated oral arguments — but also the purpose of patents.

“As we have recognized before, patent protection strikes a delicate balance between creating incentives that lead to creation, invention, and discovery,” Thomas wrote, “and “impeding the flow of information that might permit, indeed spur, invention.”

Some experts in the field said the decision provides needed clarity.

“The biotechnology industry won today, because they have some certainty,” said Kevin Noonan, a leading biotechnology and intellectual-property lawyer. “The synthetic DNA is patentable. . . . This [particular issue] is off the table. It’s not going to be challenged.”

Matthew J. Dowd, a Washington lawyer who filed a brief opposing Myriad’s position on human genes on behalf of James Watson, the scientist who co-discovered the structure of DNA, said the court’s ruling on cDNA was especially important for companies.

“The cDNA has the same genetic information as our natural gene, but it’s in a user-friendly form,” Dowd said. “If you think of a human gene as gold still in a gold mine, then cDNA is the gold after it’s been mined and polished.”

But not all were happy. Jim Greenwood, president and chief executive of the Biotechnology Industry Organization, said the decision will put U.S. companies at a disadvantage.

“The United States is now the only developed country to take such a restrictive view of patent eligibility, signaling an unjustified indifference towards our global economic and scientific leadership in the life sciences,” Greenwood said in a statement.

Those who challenged the patents said the ruling will spur research and competition. “The cost of genetic testing should come down considerably,” said Harry Ostrer of the Albert Einstein College of Medicine at Yeshiva University in New York.

Ostrer noted that there were about 4,000 gene-related patents that can now be challenged, and he predicted that additional tests for heart conditions and neuromuscular diseases will become available.

Companies already are announcing plans to offer genetic screening similar to Myriad’s test. But Ostrer and others said the company, which won the race to locate and isolate the BRCA1 and BRCA2 genes in the early 1990s, remains far ahead of the rest of the field.

Even though Noonan noted that Myriad’s patent is scheduled to expire in 2015, he added: “Myriad has a 15-year head start on everybody. They started this. They have all the relationships with everybody. They are still going to be the gold standard.”

The court’s opinion does not settle all questions about the issue.

For instance, Timothy McCaffrey, director of the division of genomic medicine at George Washington University Faculty Associates, said the emerging practice in his field is to patent a method or process of working with a gene, rather than trying to isolate and patent the gene itself.

Thomas’s opinion held out that option.

“Had Myriad created an innovative method of manipulating genes while searching for the BRCA1 and BRCA2 genes, it could possibly have sought a method patent,” Thomas wrote. “But the processes used by Myriad to isolate DNA were well understood by geneticists at the time.”

While the briefs and opinion in the case often offered theoretical arguments, reaction to the decision was more emotional.

Lisbeth Ceriani, one of the plaintiffs represented by the ACLU, said the decision means that a part of the human body is “not being held hostage by a private company anymore.”

The ruling was also praised by Rep. Debbie Wasserman Schultz (D-Fla.), chairman of the Democratic National Committee, who underwent a double mastectomy and had her ovaries removed after Myriad’s test showed she carried the gene mutation. Her treatment required seven surgeries, she said. “Because one company had a patent on the BRCA1 and BRCA2 genes, I was unable to get a second opinion,” Schultz said.

The case is Association for Molecular Pathology v. Myriad Genetics Inc.