Researchers have corrected a disease-causing gene mutation with a single infusion carrying a treatment that precisely targeted the errant gene.

This was the first time a mutated gene has been restored to normal.

The small study of nine patients announced Monday by the company Beam Therapeutics of Cambridge, Mass., involved fixing a spelling error involving the four base sequences — G, A, C and T — in DNA. The effect was to change an incorrect DNA letter to the right one. The result was a normal gene that functioned as it should, potentially halting liver and lung damage of patients with a rare disorder.

“This is the beginning of a new era of medicine,” said Dr. Kiran Musunuru, a gene therapy researcher at the University of Pennsylvania’s Perelman School of Medicine.

He added that the method offers the hope of treating other genetic diseases precisely by fixing mutations — an alternative to current gene therapies, which either add new genes to compensate for mutated ones, or slicing DNA to silence genes.

Dr. Musunuru is a co-founder and equity holder of Verve Therapeutics, a gene therapy company, and receives funding from Beam Therapeutics for research, but not for this study.

Dr. Richard P. Lifton, president of Rockefeller University and head of its Laboratory of Human Genetics and Genomics, said the sort of gene editing Beam did, rewriting genes with an infusion, “is a holy grail” that “has the promise for being a one-and-done kind of therapy.”

Dr. Lifton is a director of Roche Pharmaceuticals and its subsidiary Genentech.

Despite the study’s small size, he said the results are “a very impressive advance and very promising.”

The study involved patients who have alpha-1 antitrypsin deficiency, or AATD, a genetic disease that affects an estimated 100,000 Americans, mostly of European ancestry. That makes it as common as sickle cell in this country. It is progressive and incurable.

The alpha-1 antitrypsin protein is made in the liver and normally goes to the lungs and protects them from inflammation from inhaled irritants like smoke or dust. But in people with the disease, a single change in a DNA letter in the gene results in a misshapen and nonfunctional protein. The result is often emphysema or chronic obstructive pulmonary disease in unprotected lungs.

But many of the aberrant alpha-1 antitrypsin proteins never get to the lungs and instead build up in patients’ livers, often causing cirrhosis.

The gene editing was simple for patients. They sat in a chair for a couple of hours while lipid nanoparticles, like those used in Covid vaccines, were infused into their blood. The nanoparticles did not hold vaccines, though. Instead, they encased a microscopic gene editor. The lipid casing protected the editor on a journey to the liver.

When the nanoparticles reached the liver, the lipid layer peeled off, releasing the editor — a disabled CRISPR molecule that acted like a GPS for the genome and an enzyme to fix the mutation. The CRISPR molecule crawled along the patient’s DNA until it found the one incorrect letter that needed to be repaired among the three billion DNA letters in the genome. Then the editing enzyme replaced that letter with the correct one.

The study divided the patients into three groups and tested three different doses of the gene editor. Those who got the highest dose made enough normal alpha-1 antitrypsin to be in a range where no more damage should occur. There were no serious side effects, said John Evans, Beam’s chief executive officer.

Beam will now be offering the higher dose to the patients who got the lower doses in the company’s study. Beam will also study the treatment in more patients, and test an even higher dose of its gene editor.

“And then we immediately have to think about how we can get this approved,” Mr. Evans said.

Dr. Noel McElvaney, a professor at the Royal College of Surgeons in Ireland and an investigator in the Beam study, said there’s a real need for an effective treatment to halt the damage done by the mutated gene. He said he sees patients in their 30s and 40s with severe emphysema and “really bad liver disease.” And, he said, “by the time we see them there is already a significant amount of damage.”

For those suffering the worst effects of AATD, he said, the new gene therapy is “a major major breakthrough.”

“The big pro” of the new treatment, he said, is that “it theoretically cures the liver and lung disease in one go.”

Dr. McElvaney added, though, that “like all genetic interventions, we have to follow up for a long time to make sure it’s as good as we think it is.”

But patients now have renewed hope, said Dr. Andrew Wilson, scientific director of the Alpha-1 Foundation, an advocacy group.

“We have been dreaming of gene therapy as a treatment for this disease,” he said.

Republican leaders in Congress have directed the committee that oversees Medicaid to cut $880 billion from the next budget. They say these cuts aren’t necessarily aimed at Medicaid, the insurance program for 72 million poor and disabled Americans. The cuts could come from Medicare, for instance. But Trump has vowed not to touch that very popular program. And a sum this large can’t come from anywhere else.

The Republican process is just getting started, and we don’t yet know how lawmakers will change the program. Most Medicaid money goes to states, so the best way to think about the proposal is as a cut to state budgets. State lawmakers could react by dropping coverage, raising taxes or slashing other parts of their budget. In today’s newsletter, I’ll explain a few possible scenarios.

Who’s covered

Medicaid was designed to divide a patient’s medical bills: the federal government and the state would each pay a set share. (A state’s contribution depends on how poor it is.)

The law is precise about what Medicaid must cover — cancer screenings and kidney transplants, for instance, but not prosthetic legs — and Republicans can’t change that with a budget bill. Every state has to cover certain populations, including poor children, pregnant women, people with disabilities and patients in nursing homes who run out of money.

Most states also choose to cover an optional group that was added as part of Obamacare in 2014: anyone who earns less than a certain income (around $21,000 for a single person). Republicans want to impose a work requirement on this group for people who aren’t disabled. That idea is popular with the public but would save the federal government only around $100 billion, not enough to meet the G.O.P. target.

Bigger targets

Anything more to lower the federal government’s share would put the burden on states. And lawmakers there could deal with the problem in their own ways. They could cut optional populations like the Obamacare group. Twelve states have laws that will automatically do this if federal funding drops. If they don’t want to drop people, states can drop optional benefits, such as prescription drug coverage.

After those cuts, states face tough choices.

They could pay doctors, hospitals and nursing homes less for care. But there is a limit. If Mississippi suddenly started paying $50 for an echocardiogram instead of around $160, cardiologists might stop seeing Medicaid patients. (Many Medicaid patients already struggle to find care because the program pays doctors so little.) Cuts like these could also put some nursing homes or rural hospitals out of business.

Even so, states would still need a lot more money for Medicaid, usually their second-largest expense after education.

Where could they get it? They’d have to sacrifice other priorities. One option is to cut education. Another is to raise taxes. None of these would be required by federal legislation; it’s up to the states how they cope. That allows Republicans in Congress to say they are not cutting Medicaid benefits or eligibility, even if that is the inevitable effect in most places.

Too big to fail

Republicans point out that the original pact between Washington and the states has frayed, and feds are covering more than their share. That’s true. Through various accounting gimmicks, states have lowered their Medicaid contributions and now pay about a third of the bill, on average. Plus, Washington assumed almost the whole cost of the 2014 Obamacare expansion.

But that expansion has made Medicaid popular. More than half of Americans say someone in their family has used the program, and only 17 percent support cutting its budget. Local lawmakers also probably won’t win over voters by chopping education or raising taxes to save Medicaid. That’s why Democrats have settled on Medicaid as their top talking point about the G.O.P. budget plan.

Republicans tried to cut Medicaid’s budget in 2017, too. Grassroots opposition helped defeat the effort, as did extensive lobbying by Republican governors, who urged senators not to leave them with a huge fiscal hole.

The unpopularity of that bill — and its failure — helped Democrats retake the House the next year.

Related: Cutting Medicaid, taxing scholarships and killing invasive plants: A guide to the Republican wish list.

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