The immediate danger of intense exposure to high-energy gamma radiation is acute radiation syndrome (ARS), a result of irreversible bone marrow damage. The concept of selective shielding is based in the regenerative potential of the hematopoietic stem cells found in bone marrow. The regenerative quality of stem cells make it only necessary to protect enough bone marrow to repopulate the body with unaffected stem cells after the exposure: a similar concept which is applied in hematopoietic stem cell transplantation (HSCT), which is a common treatment for patients with leukemia. This scientific advancement allows for the development of a new class of relatively lightweight protective equipment that shields high concentrations of bone marrow to defer the hematopoietic sub-syndrome of acute radiation syndrome to much higher dosages.