ABL1 and atypical chronic myeloid leukemia, BCR-ABL1 negative: Atypical chronic myelogenous leukemia (aCML), BCR/ABL1 negative is a rare myelodysplastic/myeloproliferative neoplasm, usually manifested with hyperleukocytosis without monocytosis or basophilia, organomegaly, and marked dysgranulopoiesis.