The surplus of embryos is not clinically used or is unsuitable for implantation into the patient, and therefore may be donated by the donor with consent. If cultured mouse embryonic stem cells are injected into an early mouse embryo at the blastocyst stage, they will become integrated into the embryo and produce cells that differentiate into most or all of the tissue types that subsequently develop. If created successfully in humans, and if proven safe, ntES cells could completely eliminate tissue matching and tissue rejection problems. Bone marrow transplants also known as bone marrow grafts represent a type of stem cell therapy that is in common use. Differentiated cells can be reprogrammed to an embryonic-like state by transfer of nuclear contents into oocytes or by fusion with embryonic stem ES cells.