根据以下资料，回答题：
        Every year, 2,000 American lives are saved by the selflessness of others．These are the bone marrow donorswho give the gift of life to patients fighting deadly diseases such as leukemia, lymphoma, and aplastic anemia．That's the good news．The bad news is that thousands more die each year because not enough people have signedon to the registries that would help the ill find a suitable match for a transplant．
        Bone marrow or stem-cell transplants are usually a last resort, intended for those whose illnesses have notresponded to traditional treatments such as chemotherapy or radiation．How do they work? We all store a specialtype of cell in our bone marrow called stem cells．These primitive cells give rise to the three types of blood cells:red, white and platelets．Everyone's stem cells have certain genetic characteristics or markers that make themunique from others．Despite this uniqueness, there are Some shared characteristics between people．This isimportant, because a patient's immune system will reject blood or organs received from someone else if they do notshare sufficient similarities．
        Family members, especially siblings, are always the first to be considered as donors, because there's a greaterchance that the genetic markers on their cells will have enough in common to prevent rejection aftertransplantation．In many cases, however, a familial match can't be found and then the search begins for anunrelated donor．These donors typically come from a pool of people who have already signed up on a donorregistry in the event that their cells match a needy recipient．
        Once the lab has verified a match between donor and recipient, the next phase starts．The patient is givenradiation or chemotherapy to kill the unhealthy cells．Healthy cells are harvested from the donor-either extractedfrom the pelvic bones or taken from the ann in a way that is similar to having blood drawn--and prepared in alaboratory．Once they're ready, they're given to the patient through a vein--the same way as one would receive ablood wansfusion．Once these transplanted donor cells get settled within the patient's bone marrow, they make thehealthy red blood ceils, white blood cells and platelets necessary to support life．
        One of the major problems currently faced by transplant centers is that while bone marrow transplants can curemore than 70 different diseases, there aren't enough donors on the registry to treat the more than 3,000 patientsawaiting transplants．The National Marrow Donor Program, the largest registry in the country, has approximately4．8 million adult volunteer donors, but that isn't nearly enough for the thousands who need transplants．
        Why isn't a pool of more than 4 million donors isn't enough to cover 3,000 needy patients? Here's the reason:in the vast majority of cases, finding a suitable match isn't easy．Because we are unique individuals with a varietyof ancestral backgrounds and integration patterns, finding someone similar to us is a major task．In the end, itcomes down to a numbers game--the more potential donors listed on the registry, the greater a chance of finding amatch, especially for those with unusual genetic characteristics．

The shared genetic characteristic of stem cells is important in bone marrow transplant became __