 CML is characterized by the presence of the Philadelphia chromosome, which is the product of a reciprocal translocation between chromosomes 9 and 22 that results in the formation of BCR-ABL1.     ///    This underlines the importance of BCR-ABL1 at this stage of the disease.     ///    Apart from its diagnostic importance in CML patients BCR-ABL1 it is a potent oncogene.     ///    Clinical experience shows that long term remissions can be achieved at a high rate at least in chronic phase by specific inhibition of BCR-ABL1.     ///    "How is BCR-ABL1 organized on the genetic level, is there a genetic precursor lesion as discussed for Philadelphia-negative myeloproliferative diseases, what is its role in pathogenesis and progression of CML and what is its role in the CML-stem cell? These questions will be discussed in this review.. "