Are siblings always a match for bone marrow?

No. A full brother or sister has only about a 25% chance — roughly 1 in 4 — of being a complete HLA match for you. That is because the genes that control tissue matching are inherited together in sets, and each sibling draws a different combination from the same two parents.

Because the odds inside a family are that low, about 70% of patients who need a transplant have no matching donor in their family at all. They depend on a stranger from the public registry. That is the entire reason the registry exists, and the reason a single cheek swab from someone unrelated can matter so much.

Joining the registry is not the same as donating. Registering is a cheek swab and a short health form. You are contacted only if you turn out to be a possible match for a patient, and you can ask questions and decline before anything else happens.

The reason comes down to how tissue type is inherited. The genes that control HLA matching — the markers your immune system uses to tell your own cells from foreign ones — sit close together on the same chromosome and are passed down as a block, called a haplotype. You inherit one haplotype from your mother and one from your father. The National Cancer Institute explains that HLA matching between donor and recipient is what determines compatibility for a transplant, and that a close match lowers the risk of complications.

Each parent has two haplotypes and passes one of them to each child. That gives four possible combinations a child can inherit. Two full siblings are a complete match only when they happen to draw the same combination from the same two parents — and the math on that works out to about 1 in 4. The same logic means there is roughly a 1-in-4 chance two siblings share no haplotype at all, and a 1-in-2 chance they share exactly one. A study in Biology of Blood and Marrow Transplantation, indexed on PubMed by the National Library of Medicine, modeled these sibling-match probabilities and what they mean for how often patients need an unrelated donor instead.

This is why having more siblings raises a patient's overall odds — each additional brother or sister is another roughly 1-in-4 chance — but never guarantees a match. A patient with one sibling has a single roll of those dice. A patient with no siblings has none. And families today are often smaller than they once were, which leaves more patients looking outside the family than in generations past.

Put the sibling odds together with the reality of family size and the picture becomes clear: a family match is the exception, not the rule. About 70% of patients who need a transplant do not have a matching donor anywhere in their family, according to the federal donor program data summarized by the HRSA C.W. Bill Young Cell Transplantation Program. Those roughly 7 in 10 patients depend on someone unrelated.

That is precisely the gap the public registry is built to close. When no relative matches, the transplant team searches a worldwide pool of volunteers who have already submitted a cheek swab and had their HLA type recorded. The Fred Hutchinson Cancer Center — the institution where modern bone marrow transplantation was pioneered — describes how a transplant relies on matching the donor and recipient's tissue type, and how the team searches international registries for an unrelated volunteer donor when no family match exists.

A patient's best unrelated match is most often someone of similar ancestry, because HLA types cluster within ancestral backgrounds. That is why registries in many countries recruit broadly and why the worldwide pool, coordinated across registries as the WMDA Global Trends Report 2024 describes, matters so much. The more people from more backgrounds who register, the more of those 7-in-10 patients find their match.

This is not an abstract statistic for the Jada Bascom Foundation. Jada Bascom was diagnosed with acute myeloid leukemia at one month old. Like 70% of patients who need a transplant, no one in her family was compatible. Her only hope lay somewhere among the millions of strangers registered as potential donors around the world.

Five thousand miles away in Germany, a young man named Torsten Huber had read a newspaper article about bone marrow donation and registered through DKMS, the German registry. He turned out to be Jada's match. She received her transplant at seven months old. Today, she is a healthy young woman — cancer-free and thriving. In his own words: "Of course I registered to help somebody. I'm really glad that I could help Jada."

Torsten was a stranger when he swabbed his cheek. He had no idea a baby on another continent would one day depend on that decision. That is what the registry makes possible — and it is why a sibling-match statistic, of all things, leads straight to the most important point on this page: the patients who do not have a family match need you.

If the gap above moves you to act, it helps to know exactly what registering involves, because it is far less than most people assume. Joining the registry means ordering a kit, rubbing a cotton swab on the inside of your cheek, and filling out a short health-history form. That is it. There is no needle and no procedure to join.

You are added to the pool, and you are contacted only if you turn out to be a possible match for a specific patient. If that day ever comes, there is a second, more detailed health screening, and you can ask every question you have and still decline before anything else happens. Registering commits you to a swab, not to a donation. Our page on who can donate bone marrow walks through eligibility, which the registry confirms from the health history you provide.

Registering is free in most countries, and registries operate in many countries worldwide. Registration ages vary — many registries focus on ages 18 to 40, though some accept younger or older volunteers — and each registry makes the final eligibility call from your health history at signup.