The loss of both MBP-1 and EPX gene expression in MBP-1^−/−/EPX^−/− mice leads to a peripheral blood eosinophil deficiency that is both definitive and specific. (A) Cell counts and hematologic cell differentials showed that MBP-1^−/−/EPX^−/− double knockout mice have an eosinophil deficiency (without effects on the composition of the other prominent leukocytes; mean ± standard error of the mean; n = 5 to 8 animals per group) that was equivalent to the deficiency observed in an engineered transgenic mouse model congenitally devoid of eosinophils (ie, PHIL mice). (B) Flow cytometric assessments of circulating WBCs derived from individual mice confirm that eosinophils are virtually absent in the blood of MBP-1^−/−/EPX^−/− mice, similar to PHIL mice, and are significantly lower than eosinophil numbers observed in either wild-type controls or MBP-1^−/− or EPX^−/− single knockout animals. (C) The loss of eosinophils in MBP-1^−/−/EPX^−/− mice had no effect on basophils. Flow cytometric analysis (supplemental Figure 2) of bone marrow–derived leukocytes demonstrated again that the loss of eosinophils in MBP-1^−/−/EPX^−/− mice, similar to PHIL mice, had no effect on basophil populations. *P < .05.