ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive: In the field of MPNs, the discovery of BCR::ABL1 fusion (arising from a reciprocal translocation between chromosomes 9 and 22), which is the hallmark of chronic myeloid leukemia (CML), is an archetypal example of how the diagnosis of a fusion gene has successfully transformed clinical outcomes through the development of effective targeted therapy [9].