The loss of both MBP-1 and EPX gene expression in MBP-1^−/−/EPX^−/− mice significantly reduces the steady-state number of eosinophils and their progenitors in the bone marrow. (A) Flow cytometric assessments of bone marrow–derived leukocytes from individual mice showed that the number of eosinophils (ie, CCR3⁺ and IL-5Rα⁺ cells) is significantly lower in MBP-1^−/−/EPX^−/− mice relative to either wild-type controls or MBP-1^−/− or EPX^−/− single knockout animals. *P < .05. (B) Immunohistochemical staining (dark-staining cells) of femur sections using a rat mAb (MT3 25.1.1) recognizing Ear-1, -2, -6/7, and -5/11 confirmed our flow cytometric data showing that the number of Ear⁺ eosinophils in MBP-1^−/−/EPX^−/− mice was significantly lower than the number of eosinophils observed in the marrow of either wild-type, MBP-1^−/−, or EPX^−/− mice. However, unlike the marrow of mice devoid of eosinophils and their progenitors (ie, PHIL mice), the marrow of MBP-1^−/−/EPX^−/− mice maintained a finite steady-state population of these eosinophil lineage-committed cells. Isotype control: rat normal serum immunoglobulin G (IgG). Scale bar = 100 µm.