FVP constitutively activates JAK2 and STAT5. Whole-cell extracts of Friend virus-infected erythroblasts were immunoprecipitated with anti-JAK2 (top group) or anti-STAT5A (bottom group) antibody and Western blotted. The antibody used for the Western blot is indicated to the left of the panels. The strain of mice, strain of Friend virus, and stimulation with erythropoietin are indicated at the top.