Does a bone marrow transplant change your DNA?

Blood-forming stem cells live in your bone marrow. They make your red blood cells, white blood cells and platelets. In an allogeneic transplant, these stem cells come from a donor. Once they settle in, the white blood cells and immune cells they make carry the donor’s DNA, and the red cells and platelets they make come from the donor too.

The rest of your body doesn’t switch over. Your skin and most other tissues are still made of your own cells, with your own DNA. Someone whose cells don’t all share the same DNA is called a chimera, and this mix is called chimerism. The name comes from a creature in Greek myth made from parts of different animals.

Donor DNA can also turn up outside the blood. Saliva and swabs from the inside of the cheek are two examples, partly because saliva contains white blood cells. That matters for some DNA tests, explained below.

After a donor transplant, the care team tests blood to see how much of it comes from the donor. Labs look for small DNA differences between patient and donor, so they can tell whose cells are whose. It is one way doctors check whether the transplant is working.

Full donor chimerism means all of the blood cells come from the donor. Mixed chimerism means some come from the donor and some are still the patient’s own. Full donor chimerism is usually the aim, the UK stem cell charity Anthony Nolan explains, because it is linked with a lower chance of relapse, which means the disease coming back.

A mixed result is not automatically bad news. Anthony Nolan notes that reaching full donor chimerism can take months or years. What a result means depends on the disease and the person, so doctors may test different kinds of blood cells and keep following patients long after the first 100 days.

A patient and donor don’t need the same blood type. After a donor transplant, the patient’s blood type changes to the donor’s, NMDP explains. That happens because the donor’s stem cells now make the red blood cells. It is a normal part of a donor transplant.

If your donor is a different sex from you, your blood cells carry your donor’s sex chromosomes. A chromosome test on blood from a man with a female donor, for example, can show XX. Doctors can use this difference to follow the donor cells.

This does not change your sex. Doctors in China who reported one such case in 2025 wrote that the transplant replaced only the blood-forming stem cells and did not change the patient’s biological sex.

Many DNA tests use a cheek swab or saliva instead of blood. After a donor transplant, these samples can hold a lot of donor DNA as well as your own. Saliva contains white blood cells, and after a donor transplant those cells come from the donor.

A mixed sample can give wrong results. In a 2025 report from a hospital in the Netherlands, a cheek swab from one woman, taken about 30 years after her transplant, held an estimated 63% donor DNA. Tests on that swab gave wrong results for two of the six genes checked.

That is why genetic testing after a donor transplant may use a different sample. Options include blood saved before the transplant, if the hospital stored some, hair roots, or a small skin sample grown in a lab. Researchers are still comparing how reliable each one is.

The same problem can affect home ancestry tests and DNA used to identify a person, such as in forensic cases. One major ancestry-testing company advises people who have had a bone marrow or stem cell transplant not to take its saliva test, because results may be unclear or may even show the donor’s DNA.

A donor transplant replaces the blood-forming system, not the rest of you. Your skin and most other tissues keep your own DNA. Some people notice changes in taste or personality, but Anthony Nolan says there’s no medical evidence linking these to the donor’s cells.

Treatment can still change how you look and feel for a while. The US National Cancer Institute lists hair loss, skin reactions and tiredness among possible short-term problems. The new immune system can bring changes too, such as new allergies, Anthony Nolan notes.

Your children get your DNA, not your donor’s. Only eggs and sperm pass genes on to children, and Anthony Nolan explains that there’s no way for donor DNA to get into them. Transplant treatment can affect fertility, though, which is a separate question for the care team.

Donating doesn’t change the donor’s DNA either. The donor gives cells but receives none back. One ancestry-testing company notes that being a bone marrow or stem cell donor has no effect on its results.

In an autologous transplant, your own stem cells are collected before strong treatment and given back afterward. Because the cells are yours, they match, and no one else’s DNA is added.

Chimerism, a possible change of blood type and donor DNA in test samples come with allogeneic transplants. Those use cells from another person, such as a relative or an unrelated donor, or from donated umbilical cord blood.