Additional sex comb-like 1 ( ASXL1) is an enhancer of Trithorax and Polycomb family, which are necessary for the maintenance of stable repression of homeotic and other loci. Recently, alterations of ASXL1 gene were identified in the hematopoietic cells from patients with a variety of myeloid malignancies, including chronic myelomonocytic leukemia (CMML, 43% of cases), myelodysplastic syndrome (MDS, 20%), myeloproliferative neoplasms (MPN, 10%) and acute myeloid leukemia (AML, 20%). The majority of ASXL1 mutations are frameshift and nonsense mutations. These clinical data suggest an important role of ASXL1 in the pathogenesis and/or transformation of myeloid malignancies. However, the role of ASXL1 in the pathogenesis of myeloid malignancies and in normal hematopoiesis in vivo, as well as the underlying mechanisms remains unknown. This article reviews the structure and function of ASXL1, the clinical characteristic and prognostic significance of ASXL1 mutation, the association of ASXL1 with other gene mutation, as well as ASXL1 knock-down or silence in vitro and in vivo models.