FAB classifies MDS into five types with the following criteria: I. Refractory anemia (RA) Blood picture: anemia, occasionally patients with granulocytopenia and thrombocytopenia without anemia. Reticulocytes are reduced. Erythrocytes and granulocytes with pathological hematopoiesis. Primocytes absent or <1%. Liu Xueyong, Department of Hematology, Langfang Hospital Bone marrow: active or markedly active proliferation. The red lineage is hyperplastic and has pathological hematopoiesis. Rarely granulocytic and megakaryocytic pathological hematopoiesis is seen. Primitive cells <5%. Refractory anemia with ring iron granulocytes (RAS): The number of ring iron granulocytes in the bone marrow is 15% or more of all nucleated cells in the bone marrow, otherwise the same as RA. Refractory anemia with primitive cellularity (RAEB) Blood picture: 2 lineages or complete blood cells are reduced, mostly granulocytes with pathological hematopoiesis, primitive cells <5%. Bone marrow: hyperplasia is obviously active, both granulocytic and erythrocytic lineages are hyperplastic, all 3 lineages have pathological hematopoiesis, primitive cell type I + type II is 5% to 20%. Chronic granulocytic mononuclear leukemia (CMML) Blood picture: absolute value of monocytes > 1×109/L. Granulocytes are also increased and there is granulocytopenia or Pelger-Huet abnormality. Primitive cells <5%. Bone marrow: same as RAEB, primitive cells 5%-20%. RAEB in transformation (RAEB-T): Blood and bone marrow resemble RAEB, but with any of the following three conditions: 1. 75% primitive cells in blood; 2. 20-30% primitive cells in bone marrow; 3. Naïve cells with Auer vesicles.