Chorionic Villus Sampling: A procedure in which a small sample of cells are removed from the placenta where it joins the uterus. Used to test for chromosome abnormalities such as Down syndrome in the fetus.
Graft-Versus-Host Disease: A reaction of donated bone marrow against a patient's own tissue. Also called GVHD.
Mutation: Any change in the DNA of a cell. Mutations may be caused by mistakes during cell division, or they may be caused by exposure to DNA-damaging agents in the environment. Mutations can be harmful, beneficial.