While retrovirally enforced expression of BCR-ABL in hematopoietic stem (Kit⁺Sca⁺Lin⁻ [KSL]) cells initiates and drives development of chronic-phase CML, the transition to blast crisis is promoted by Hes1-mediated prevention of common myeloid (CMP) or granulocyte macrophage (GMP) progenitor (purple) differentiation.