Medicines that change our genes have moved from science fiction to the pharmacy in less than a decade. The terms can be confusing, though: gene therapy, gene editing and CRISPR are often used as if they meant the same thing. They do not. This guide explains the difference, what is already available, and what still holds these treatments back.

The basic idea

Many diseases are caused by a faulty gene, a stretch of DNA that carries the instructions for making a protein. Conventional drugs manage the consequences. Genetic medicines go after the instructions themselves, with the aim of a one-time, long-lasting treatment. There are two main ways to do it.

Gene therapy: adding a working copy

Classic gene therapy does not fix the broken gene. It delivers an extra, working copy so cells can make the missing protein. The copy is carried into cells by a vector, usually a harmless virus stripped of its own genes; the most common is the adeno-associated virus (AAV). Approved examples include Luxturna for an inherited form of blindness, Zolgensma for spinal muscular atrophy in infants, and Hemgenix for hemophilia B. The original faulty gene stays where it is.

Gene editing: rewriting the DNA

Gene editing changes the DNA already in a cell. It can switch a gene off, correct a misspelling, or adjust a genetic control switch. The change is made in the cell’s own genome, so it is copied whenever the cell divides. Because it is so precise, editing can do things that adding a gene cannot — such as silencing a harmful gene.

So what is CRISPR?

CRISPR is the most widely used gene-editing tool. It was adapted from a defense system bacteria use against viruses, work that earned Jennifer Doudna and Emmanuelle Charpentier the 2020 Nobel Prize in Chemistry. It has two parts: a guide RNA, a short sequence that matches the target stretch of DNA and acts like a search term, and an enzyme, usually Cas9, that cuts the DNA at that spot. The cell’s repair machinery then seals the cut, typically disabling the gene. Changing the guide RNA retargets the system, which is what made CRISPR so much faster and cheaper than earlier editing tools.

Newer, gentler editors

Cutting both strands of DNA is effective but blunt. Two refinements avoid a full cut. Base editing chemically converts a single DNA letter into another, like correcting a typo. Prime editing can rewrite short stretches, a “search and replace.” In 2025, doctors in Philadelphia used a custom-made base editor to treat an infant with a rare, life-threatening metabolic disorder, designing and delivering the therapy within months — a first glimpse of personalized gene editing.

Inside the body or outside it

Genetic medicines are delivered in two ways. Ex vivo means cells are taken out of the body, modified in a laboratory, and returned. In vivo means the treatment is infused or injected and does its work inside the body, often carried to the liver by lipid nanoparticles. In vivo treatment is simpler for patients, but getting editors safely to organs other than the liver remains one of the field’s biggest challenges.

What is approved

The first CRISPR medicine, Casgevy, was approved in late 2023 for sickle cell disease and beta thalassemia. It is an ex vivo treatment: a patient’s blood stem cells are edited to switch on fetal hemoglobin, a form of the oxygen-carrying protein normally made only before birth, which compensates for the faulty adult version. Most treated patients have been freed from pain crises or transfusions. In vivo editing is in trials for conditions including high cholesterol; a single infusion of one experimental treatment halved LDL cholesterol for a year in an early study.

What about RNA medicines?

A third category is often confused with these. RNA-based drugs, such as siRNA and antisense medicines, block the messages that genes send without touching the DNA. Their effect wears off, so they are given by injection every few months. They are not gene therapy or gene editing, but they can treat some of the same diseases, reversibly.

The risks

  • Off-target edits: an editor may change DNA in the wrong place, with a theoretical risk of cancer.
  • Immune reactions: high doses of viral vectors have caused serious liver injury and, rarely, deaths.
  • Harsh preparation: ex vivo treatments like Casgevy require chemotherapy to clear the bone marrow, which carries its own risks, including infertility.
  • Uncertain durability: some gene therapies fade over time, and AAV treatments generally cannot be given twice.
  • Permanence: an edit cannot be undone, which is why regulators ask for up to 15 years of follow-up.

What they cost

These are among the most expensive medicines ever sold. One-time list prices run from about $2 million to $3.5 million: roughly $2.1 million for Zolgensma, $2.2 million for Casgevy and $3.5 million for Hemgenix. Makers argue that a single treatment replaces a lifetime of costly care. Insurers and governments are experimenting with payment by installment and refunds if a treatment fails, but access remains uneven, and several gene therapies have been commercial disappointments despite working.

A line the field does not cross

All approved and experimental treatments edit somatic cells, the ordinary cells of the body, so changes are not inherited. Editing eggs, sperm or embryos — germline editing — would pass changes to future generations. It is prohibited or tightly restricted in most countries, and there is broad scientific agreement that it should not be used clinically.

The bottom line

Gene therapy adds; gene editing rewrites; CRISPR is the leading tool for rewriting. Both have produced real treatments for diseases that had none, mostly rare and severe ones so far. Whether they reach common conditions will depend on safer delivery, lower cost and long-term evidence. For individual patients, eligibility is specific to the disease and often to the exact mutation, so the place to start is a specialist or genetic counselor. This explainer is general information, not medical advice.