Exciting new research has confirmed the presence of pre-leukaemic stem cells in the bone marrow of identical twins - one with B-cell acute lymphoblastic leukaemia, one without (but who now can be considered to be at risk). The abnormal stem cells result from the fusion of two genes during pregnancy to form a hybrid protein "TEL-AML 1". They either remain dormant or develop into leukaemic stem cells.
The Leukaemia Research Fund press release ishere. Unfortunately, the original paper in Science is not free.