Children receiving half-matched (haploidentical) transplants from a parent are most likely to benefit from this strategy. At present these patients receive blood stem cells from a parent, but the T cells are removed because the risk of serious GVHD is unacceptable. This means that they are much more likely to suffer from life threatening infections or experience a relapse of leukaemia. The investigators want to use gene therapy to produce "safe" T cells which can be used to strengthen the transplant and prevent these serious complications.