The Human Leukocyte Antigens need to be perfectly matched in usual stem cell transplants. These antigens are present in white cells and are responsible for rejection or Graft Versus Host Disease (GVHD). Matching refers to the proteins called Human Leukocyte Antigens (HLA) that appear on the surface of white blood cells and other tissues in the body. These six HLA points, or loci, determine tissue compatibility between a patient and a donor. Although a perfect match would be best, studies have shown that cord blood transplants can be successful, even when only four of the six loci match. With cord blood, the immune cells are less mature than those in bone marrow, and therefore siblings are twice as likely to be able to use each other’s cord blood, compared to bone marrow.