Scientists have discovered the cancer ‘stem cells’ that cause acute lymphoblastic leukaemia, the most common form of childhood cancer. The breakthrough came through studying four-year-old identical twins Olivia, who has leukaemia, and Isabella, who is healthy. They found that both twins had abnormal ‘pre-leukaemia’ stem cells in their blood that can either lie dormant in the bone marrow or develop into full-blown leukaemia stem cells. The results were then confirmed with experiments using human cord blood cells.