Inclusion Criteria:

Men and women ≥ 18 years of age
Documented diagnosis of myelodysplastic syndromes (MDS) or non-proliferative chronic myelomonocytic leukemia (CMML), white blood cells (WBC) ≤ 13,000 /mm^3, World Health Organization (WHO) that meets International Prognostic Scoring System (IPSS) criteria for low or intermediate-1 risk disease
Anemia, Hemoglobin (Hgb) ≤ 9.0 g/dL or ≥ 2 units of Red Blood Cells (RBCs) within 84 days
No response or loss of response to Erythropoiesis-Stimulating Agents (ESAs) or erythropoetin (EPO) > 500 mU/ml
Eastern Cooperative Group (ECOG) score ≤2.
Creatinine < 1.5 * Upper Limit of the Normal (ULN)
Total bilirubin ≤3.0 mg/dL
Aspartate aminotransferase (AST)/Serum glutamic oxaloacetic transaminase (SGOT) & Alanine Aminotransferase (ALT)/Serum Glutamic Pyruvic (SGPT) ≤3.0 * Upper Limit of Norma (ULN)
Free of metastatic malignancy (other than MDS) for ≥2 years
Highly effective methods of birth control for females and males

Exclusion Criteria:

Chromosome 5q deletion
Pregnant or breast feeding women and males who do not agree to use condom during the sexual contact with females of childbearing potential
Major surgery within 30 days
Incomplete recovery or incomplete healing of wounds from previous surgery
Heart failure ≥3 (New York Heart Association (NYHA))
Thromboembolic or myocardial infarction event within 6 months
Concurrent anti-cancer cytotoxic chemotherapy
History of severe allergic or anaphylactic reaction or hypersensitivity to recombinant protein
Known positive for Human Immunovirus (HIV) or infectious Hepatitis type C or active infectious Hepatitis type B
Clinically significant anemia unrelated to MDS
Thrombocytopenia (<30,000/uL)
Uncontrolled hypertension
Treatment with another investigational drug or device within 28 days prior to Day 1
Prior exposure to sotatercept (ACE-011)
Any serious medical condition, lab abnormality or psychiatric illness