Chronic myelomonocytic leukaemia (CMML) is a clonal haematopoietic malignancy characterized by persistent monocytosis (>1 × 109/L), myeloid cell proliferation with myeloid cell dysplasia, ineffective haematopoiesis, and a propensity to transform into acute myeloid leukaemia (AML). It is a very heterogeneous disease, with haematological characteristics ranging from those of a myelodysplastic syndrome (MDS) with peripheral monocytosis, to very proliferative forms, characterized by high WBC counts, splenomegaly, and/or other forms of extramedullary disease.