Umbilical cord blood was once discarded as waste material but is now known to be a useful source of blood stem cells. After a baby is born, cord blood is left in the umbilical cord and placenta. It is easy to collect, with no risk to the mother or baby. It contains haematopoietic (blood) stem cells: rare cells normally found in the bone marrow. Haematopoietic stem cells (HSCs) can make every type of cell in the blood – red cells, white cells and platelets. They have been used for many years in bone marrow transplants to treat blood diseases. Cord blood is used to treat children with cancerous blood disorders such as leukemia, or genetic blood diseases like Fanconi anemia. The cord blood is transplanted into the patient, where the HSCs can make new, healthy blood cells to replace those damaged by the patient's disease or by a medical treatment such as chemotherapy for cancer. In this way, cord blood offers a useful alternative to bone marrow transplants for some patients. It also seems to be less likely than bone marrow to cause immune rejection or complications such as Graft versus Host Disease. However, cord blood transplants also have limitations. Treatment of adults with cord blood has so far proved to be very challenging, despite some successes. Cord blood can also only be used to treat blood diseases. No therapies for non-blood-related diseases have yet been developed using HSCs from either cord blood or adult bone marrow. A major limitation of cord blood transplantation is that the blood obtained from a single umbilical cord does not contain as many haematopoeitic stem cells as a bone marrow donation. A transplant containing too few HSCs may fail or could lead to slow formation of new blood in the body in the early days after transplantation. This has been partially overcome by transplanting blood from two umbilical cords into larger children and adults. Some researchers have also tried to increase the total number of HSCs obtained from each umbilical cord by collecting additional blood from the placenta. Much research is focused on trying to increase the number of HSCs that can be obtained from one cord blood sample by growing and multiplying the cells in the laboratory. This is known as "ex vivo expansion". Several preliminary clinical trials using this technique are underway. The results so far are mixed: some results suggest that ex vivo expansion reduces the time taken for new blood cells to appear in the body after transplantation; however, adult patients still appear to need blood from two umbilical cords. Neither solution is entirely satisfactory.