These proteins are on the surface of most tissue cells and give an individual his or her unique tissue type. HLA factors are inherited from mother and father and the greatest chance of having the same HLA type is between siblings. On average one in four siblings is expected to share the same HLA type. The testing for HLA antigens is referred to as “tissue typing.” There are six major groups of HLA antigens A, B, C, D, Dr, and Dq. These proteins on the surface of cells act as antigens when donated (transplanted) to another individual, the recipient. If the antigens on the donor cells are identical (e.g. identical twins) or very similar (e.g. HLA matched-sibling) the transplant (donated stem cells) are more likely to survive (engraft) in the recipient. In addition, the recipient's body cells are less likely to be attacked by the donated immune cells (graft versus host disease).