The concomitant loss of MBP-1 and EPX led to a targeted reduction in EoP cells without similar effects on other less committed myeloid progenitor populations. (A) Hematopoietic differentiation pathways leading to eosinophils in mice (adapted from Lee et al³ ). The unique and specific loss of eosinophils displayed by MBP-1^−/−/EPX^−/− mice suggested the blockade in eosinophilopoiesis (?) occurred at the EoP stage as opposed to earlier myeloid progenitors (ie, common myeloid progenitors [CMP] or granulocyte monocyte progenitors [GMP]). (B) Flow cytometric assessments of marrow leukocytes confirmed that the defect in eosinophilopoiesis observed in MBP-1^−/−/EPX^−/− mice is limited to the EoP population with no observed effects on either the CMP or GMP populations. (C) Flow cytometric assessments showed that the EoP populations of both PHIL (n = 3) and MBP-1^−/−/EPX^−/− (n = 7) mice each displayed significant increases in apoptosis (eightfold and threefold increases, respectively) relative to wild-type (n = 7). *P < .05; **P < .002.