As early findings consisted chiefly of isolated case reports or small case series, CNL was born of an era of being defined primarily by what it was not: by excluding basophilia, monocytosis, or a BCR-ABL fusion, it could be distinguished from chronic myeloid leukemia (CML), atypical chronic myeloid leukemia (aCML), and chronic myelomonocytic leukemia (CMML). This evidence concerns the gene ABL1 and atypical chronic myeloid leukemia, BCR-ABL1 negative.