Post-Transplant Cyclophosphamide

Background: Severe aplastic anemia (SAA), and myelodysplastic syndrome (MDS), and paroxysmal nocturnal hemoglobinuria (PNH) cause serious blood problems. Stem cell transplants using bone marrow or blood plus chemotherapy can help. Researchers want to see if using peripheral blood stem cells (PBSCs) rather than bone marrow cells works too. PBSCs are easier to collect and have more cells that help transplants. Objectives: To see how safely and effectively SAA, MDS and PNH are treated using peripheral blood hematopoietic stem cells from a family member plus chemotherapy. Eligibility: Recipients ages 4-55 with SAA, MDS or PNH and their relative donors ages 4-75 Design: Recipients will have: * Blood, urine, heart, and lung tests * Scans * Bone marrow sample Recipients will need a caregiver for several months. They may make fertility plans and a power of attorney. Donors will have blood and tissue tests, then injections to boost stem cells for 5-7 days. Donors will have blood collected from a tube in an arm or leg vein. A machine will separate stem cells and maybe white blood cells. The rest of the blood will be returned into the other arm or leg. In the hospital for about 1 month, recipients will have: * Central line inserted in the neck or chest * Medicines for side effects * Chemotherapy over 8 days and radiation 1 time * Stem cell transplant over 4 hours Up to 6 months after transplant, recipients will stay near NIH for weekly physical exams and blood tests. At day 180, recipients will go home. They will have tests at their doctor s office and NIH several times over 5 years.