Loss of ERK1 increases the response to erythropoietic stress. Five wild-type (□) and 6 ERK1^−/− (○) mice were injected with PHZ on days 0, 1, and 3. Hematocrits (A) and reticulocyte counts (B) were assessed on days 0, 3, 6, 9, and 13. Data are mean ± SD (*P < .05; **P < .01; ***P < .001).