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The hypoxic environment at high altitude stimulates erythropoiesis (top bubble). Flow cytometry was used to analyze the consequent response of erythroid precursors in vivo within the spleen and bone marrow erythroblastic island niche (middle bubbles). Two regulators of the mitochondrial apoptotic pathway, Bcl-xL and Bim, had markedly different dynamic responses to erythropoietin receptor activation (EpoR, lowest bubble). The proapoptotic Bim was suppressed for the duration of stress. By contrast, the antiapoptotic Bcl-xL and its upstream regulator, Stat5, showed classical adaptation, similar to that of sensory systems or bacterial chemotaxis. Adaptation extends the dynamic range of the stress response. Illustration by Miroslav Koulnis. For details, see the article by Koulnis et al on page 1228.