In an optimal HLA match, the patient and donor share all eight or 10 HLA (human leukocyte antigen) markers (genes or proteins) tested. This is ideal for bone marrow or stem cell transplantation because it minimizes risks of rejection or graft-versus-host disease. New techniques allow effective transplantation using stem cells from a donor who matches the patient in only seven of eight or nine of 10 HLA markers—a minimally mismatched donor. Using these donors allows many more people to benefit from a potentially life-saving transplant.