ABL1 and leukemia: Progression of BCR-ABL+ leukemia from the chronic phase to the poor prognosis blast crisis phase is accompanied by increased BCR-ABL expression, genetic instability, increased proliferation, reduced apoptosis, and a blockade of differentiation where myeloid or lymphoid progenitors/precursors fail to differentiate, resulting in the development of acute myelogenous leukemia (AML) or B-cell acute lymphoblastic leukemia (B-ALL)2–5.