Injected hRBCs survive longer in HMOX-1^–/– mice than in HMOX-1^+/+ mice. Human RBCs were injected intraperitoneally into nonengrafted HMOX-1^+/+ (n = 7) and HMOX-1^–/– MSRG47 mice (n = 7). Mice were bled retro-orbitally at days 1, 2, 6, and 8 after injection and analyzed for human RBCs (hCD235a⁺) vs mouse RBCs (Ter-119⁺). (A) Number of hRBCs per microliter of blood. (B) Total RBCs (human and mouse) per microliter of blood.