The study consists of up to four parts: Part A, to investigate the safety and tolerability of MGTA-145; Part B, to investigate the safety and tolerability of MGTA-145 when administered in combination with plerixafor; Part C, to investigate the safety and tolerability of two sequential days of dosing MGTA-145 in combination with plerixafor; and Part D, to investigate the safety, tolerability, and measure by apheresis, the total number of CD34+ cells mobilized after a dose of MGTA-145 administered in combination with plerixafor.