ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: Regarding the formation of the fusion gene in Ph negative BCR/ABL1 positive CML patients, Morris et al [6] suggested two possible mechanisms: a one step model, where BCR/ABL1 results from a simple insertion of either 3' ABL1 into BCR or 5' BCR into ABL after three genomic breaks; and a multiple step model, with an initial classical t(9;22)(q34;q11) followed by a second translocation of both products and/or a third chromosome, requiring a minimum of 4 genomic breaks.