Juvenile IRP41^−/− mice present with splenomegaly, extramedullary hematopoiesis, and increased Epo expression. WT, IRP1^−/−, and IRP2^−/− mice (n = 7) were euthanized at the age of 4 to 6 weeks. (A) Spleen size. (B) Spleen to body weight ratios. (C) Hematoxylin and eosin staining of spleen sections; magnifications at (left) 20× and (right) 40×. (D) Epo mRNA expression in the kidneys (n = 5). (E) Serum Epo levels (n = 5). ***P < .001.