A Safety and Efficacy Study to Evaluate Luspatercept in Subjects With Myeloproliferative Neoplasm-associated Myelofibrosis Who Have Anemia With and Without Red Blood Cell-transfusion Dependence

Subjects satisfying the eligibility criteria will be assigned to 1 of the following cohorts (which are enrolling in parallel) based on their eligibility: * Cohort 1 (subjects with anemia only that are not currently receiving RBC transfusions) - Cohort 2: (subjects that are RBC-transfusion dependent) * Cohort 3A: (subjects on ruxolitinib as part of their standard of care therapy that have anemia only) * Cohort 3B: (subjects on ruxolitinib as part of their standard of care therapy that are RBC-transfusion dependent) Overall, the study will enroll approximately 100 subjects worldwide.