TPO is the major Erk activator in HSPCs. (A) One representative experiment, out of 3 similar experiments, of Erk activity obtained by capillary isoelectric-focusing immunoassay in WT and Mpl^−/− LSK cells cultured for 1 hour in complete medium (Comp) or without TPO. Results show the area peak value of phosphorylated Erk1 or Erk2, normalized to the area value of total Erk. Experiment performed with cells pooled from 3 to 5 mice. (B-C) Western blot analysis of Erk activation in LSK or LSK-Flt3⁻ cells cultured for 45 or 90 minutes as indicated with SCF, FL, or TPO or in the absence of cytokine (−) and then irradiated (2 Gy) or not. Analysis was performed 30 minutes after IR. NIR, no IR.