Subjects will provide informed consent prior to undergoing any study-related procedures. Screening procedures are to take place within 28 days prior to the initiation of azacitidine treatment (Day 1, Cycle 1). Subject eligibility will be based on central pathology review. Bone marrow aspirate and bone marrow biopsy will be collected at screening and sent for morphological assessment by a central pathology reviewer prior to the subject receiving IP. The central pathology reviewer will document the MDS classification according to the FAB and WHO criteria (Appendix A and B, respectively).