1. Subjects who have undergone hematopoietic stem cell transplant (HSCT) within 60 days of the first dose of AG-221, or subjects on immunosuppressive therapy post HSCT at the time of screening, or with clinically significant graft-versus-host disease (GVHD). (The use of a stable dose of oral steroids post GVHD and/or topical steroids for ongoing skin GVHD is permitted with Medical Monitor approval.)