Pronounced splenomegaly and development of human T-ALL in NSG recipients of 56Fe ion-irradiated human HSC: At the time of euthanasia, 5 of the 14 NSG mice that were repopulated with human HSC that had been exposed to a mission-relevant dose of 56Fe ions (20 cGy, 1 GeV/n) exhibited marked splenomegaly. (a) The spleen from one of these mice, being compared with the spleen from a control mouse transplanted with sham-irradiated human HSC. This particular recipient's spleen was ∼ 30 times the normal volume; (b) H&E staining of the spleen from one of the mice transplanted with 56Fe ion-exposed human HSC, displaying overpopulation with a monotonous, rounded, lymphoid-like cell population; (c) Staining of the spleen from this mouse with anti-human CD3. (d) Staining of the spleen from this mouse with anti-human CD7; (e) Staining of the spleen from this mouse with anti-TdT; (f) Absence of any staining in the spleen from an untransplanted (negative control) NSG mouse with anti-human CD3.