• Age 3 years to 75 years

• Confirmed diagnosis of acquired SAA defined as:

• a. Bone marrow cellularity \< 25% or variable marrow cellularity but with \< 30% residual hematopoietic cells deemed HYPOcellular for age AND b. Two (2) out of 3 of the following (in peripheral blood). i. Neutrophils \< 0.5 x109/L ii. Platelets \< 20 x109/L iii. Reticulocyte count \< 20 x109/L (\< 60 x 109/L using an automated analysis)

• No suitable fully matched related donor as per Investigator's discretion (6/6 match for HLA A and B at intermediate or high-resolution and DRB1 at high-resolution using deoxyribonucleic acid \[DNA\]-based typing) available.

• Available donor as defined in the protocol.

• Participant and/or legal guardian must sign informed consent.

• Adequate organ function defined by institutional transplant standards or defined as below:

‣ Cardiac: Left ventricular ejection fraction (LVEF) at rest \> 40% with no clinical signs of cardiac failure. For participants aged \< 13 years, shortening fraction (SF) ≥ 26% by echocardiogram or multigated acquisition (MUGA) may be substituted for LVEF.

⁃ Hepatic: Total bilirubin \< 2.0 mg/dL unless Gilbert's disease is present

⁃ Renal: For participants \> 13.0 years of age at the time of enrollment: estimated creatinine clearance (CrCl) \> 60 mL/minute (per institutional standard). For participants \< 13.0 years of age at enrollment: glomerular filtration rate (GFR) estimated by the updated Schwartz formula ≥ 90 mL/min/1.73 m2. If the estimated GFR is \< 90 mL/min/1.73 m2, then renal function must be measured by 24-hour creatinine clearance or nuclear GFR, and must be \> 50 mL/min/1.73 m2.

⁃ Pulmonary:

• i. For participants \> 13.0 years of age: Diffusing capacity of the lung for carbon monoxide (DLCO, corrected/adjusted for hemoglobin \[Hb\]) \> 50%, or Spirometry with forced expiratory volume 1 (FEV1) \> 50% predicted (without administration of bronchodilator) and forced vital capacity (FVC) \> 50% predicted.

• ii. For participants \< 13.0 years of age unable to perform pulmonary function tests (PFTs) due to age or developmental ability: (1) no evidence of dyspnea at rest and (2) no need for supplemental oxygen and (3) O2 saturation \> 92% on room air at sea level (with lower levels allowed at higher elevations per established center standard of care \[e.g., Utah, 4,200 feet above sea level, does not give supplemental oxygen unless below 90%\]).

• Karnofsky or Lansky performance status ≥ 60%.

• Females and males of childbearing potential must agree to practice 2 effective methods of contraception at the same time or agree to abstinence.