Stem cells are created in the bone marrow. They mature into different types of blood cells that people need including red blood cells which carry oxygen around the body, white blood cells which help fight infections, and platelets which help the blood to clot and prevent bleeding. For a matched unrelated stem cell transplant, stem cells are collected from a person (donor) who is not related to the patient but who has the same type of stem cells. For a mismatched related stem cell transplant, stem cells are collected from a donor who is related to the patient and whose stem cells are almost the same as those of the patient but not exactly. The patient then receives high dose chemotherapy. This chemotherapy kills the stem cells in the patient's bone marrow. Stem cells that have been collected from the donor are then given to the patient to replace the stem cells that have been killed.