Analysis of ERK1^−/−-induced splenomegaly. (A) Age- and sex-matched ERK1^−/− (□) and ERK1^+/+ (■) mice at 2 to 4 months of age were killed, and spleen lengths and weights were measured. A representative comparison of ERK1^+/+ and ERK1^−/− spleens is shown in the inset (ruler increments are in millimeters). Data are means ± SEMs, n = 14 for ERK1^+/+ and for ERK1^−/− mice. (B) Splenocytes from ERK1^+/+ and ERK1^−/− mice at 2 to 4 months of age were counted on a hemocytometer. Data are means ± SEMs (n = 10) of cell number per spleen. (C) Perl Prussian blue staining for ferric iron in the spleen of 2-month-old ERK1^+/+ and ERK1^−/− mice (middle, original magnification ×100, with 10×/0.5 NA objective; bottom, original magnification ×200, with 20×/0.5 NA objective). Scale bars are for 50 μm. ***Significant differences between genotypes with P < .001.