Study to Assess the Efficacy and Safety of the Epigenetic Modifying Effects of CC-486 (Oral Azacitidine) in Combination With Fulvestrant

This is a Phase 2, open-label, two-arm study assessing the efficacy and safety of the combination of fulvestrant with CC-486 in subjects with ER+, HER2- metastatic breast cancer who have progressed after prior AI. Approximately 92 participants will be enrolled and assigned randomly in a 1:1 ratio to one of two treatment arms: * Arm A: CC-486 300 mg and fulvestrant 500 mg: 46 subjects * Arm B: Fulvestrant 500 mg: 46 subjects Each cycle will be 28 days. CC-486 will be administered orally at a dose of 300 mg daily on days 1-21 of each 28-day cycle. Fulvestrant will be administered by intramuscular (IM) injection at a dose of 500 mg on days 1 and 15 of cycle 1 and day 1 of subsequent cycles. Safety will be evaluated by an independent data monitoring committee (DMC) after a total of approximately 32 subjects have completed at least 1 treatment cycle.