Reducing the levels of Mcl-1, irrespective of JAK2 mutation, sensitizes leukemia cells to ABT-263 (Fig. 1H-I), indicating that Bcl-2 family proteins, such as Bcl-xL and Bcl-2, are necessary to maintain viability when Mcl-1 levels are reduced. This evidence concerns the gene MCL1 and leukemia.