If cells are removed from the embryo they will differentiate into any cell type. These are called embryonic stem cells. Some stem cells remain in the bodies of adults as adult stem cells.

Embryonic stem cells can differentiate into a wider range of cell types, but are difficult to obtain and their use raises ethical challenges. The best source is the five-day-old embryo.

Seb Murphy has undergone chemotherapy, immunotherapy, radiotherapy, a stem cell transplant and surgery to treat acute lymphoblastic leukaemia (ALL). But the 16-year-old successfully scored a 9 ...