5 Day Versus 7 Day Azacitidine in Lower Risk Myelodysplastic Syndrome

Inclusion Criteria: * Subjects must satisfy the following criteria in order to be enrolled in this clinical trial: Patients who have been diagnosed with MDS by the FAB criteria and belong to Low or INT-1 risk by the IPSS classification will be enrolled in this study. For the purpose of analysis, chronic myelomonocytic leukemia (CMML) patients with less than 5% of myeloblasts are also classified by the IPSS risk classification. Secondary or treatment-related MDS is allowed, but recurrent or persistent MDS after stem cell is not applicable. The enrolled patients should have anemia (hemoglobin \< 10.0g/dL), transfusion dependence, thrombocytopenia (less than 100×10\^9/L), or absolute neutrophil count less than 1.80×10\^9/L. * 18 years of age or older * Life expectancy of at least 12 months * ECOG performance status 2 or less * Serum creatinine less than 1.5 times the upper limit of normal (ULN) level of the investigating institution * Serum bilirubin less than 2.0 times the upper limit of normal (ULN) level of the investigating institution * AST, ALT, and alkaline phosphatase less than 3 times the upper limit of normal (ULN) level of the investigation institution * Patients who can have informed consent and signed the informed consent form * Male patients who have a female partner of childbearing potential must agree to use two types of effective contraceptive methods during the study and for 30 days following the last dose. * Females of childbearing potential (FCBP) must satisfy the following criteria: must agree to use the contraceptive method (oral contraceptives, injectables, hormonal implants; tubal ligation; intra uterine device; spermicidal contraceptives, the sterilized partner) approved by the physician during azacitidine treatment and for 3 months following the last dose, and must have a negative result of serum pregnancy test that was performed within 72 hours prior to starting study drug therapy. Exclusion Criteria: * Any coexisting major illness or organ failure * HIV positive, or active hepatitis B or C infection * Uncontrolled acute infection * Uncontrolled hemorrhage * Pregnant or lactating * Known or suspected hypersensitivity to azacitidine * Patients diagnosed with malignant hepatic carcinoma or malignant disease within the past 12 months (except in situ carcinoma without complication, cervical or breast intraepithelial neoplasia, or other local malignant carcinoma that is likely to be treated by surgical removal or radiotherapy)