Complex sentence: Most research on MRD has been done on leukaemia, mainly two types: adult chronic myeloid leukaemia, and childhood acute lymphoblastic leukaemia (the commonest childhood cancer).
Split into simpler sentences: Most research on MRD has been done on leukaemia. Mainly two types: adult chronic myeloid leukaemia, and childhood acute lymphoblastic leukaemia (the commonest childhood cancer).