Successful bone marrow transplants require that the donor and recipient match at their HLA (human leukocyte antigen) genes. HLA proteins are located on the plasma membranes of all cells in the body. There are 3 HLA genes that must be matched: HLA-A, HLA-B, and HLA-C. These genes are highly variable (polymorphic) in the human population; over 2,000 variant alleles are known for each of these genes. The occurrence of matching unrelated donors is at best on the order of 1 in millions, for people with relatively common HLA alleles. Usually, siblings are the most likely to match. Let's think about why this is so. Considering all three genes (HLA-A, HLA-B, and HLA-C) how many different HLA proteins would you expect the typical person to have on the surface of each of their cells?

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