TpoR and JAK2 are required in CALR mutant–induced spontaneous growth of megakaryocytic progenitors in patients. CD34⁺ cells from patients carrying CALRdel52 or CALRins5 mutations were purified. (A) The percentages of inhibition of MPL/TpoR and JAK2 mRNA expression level by quantitative real-time polymerase chain reaction in CD34⁺ progenitors transduced with lentivirus expressing either sort hairpin RNAs (shRNA) targeting MPL/TpoR or JAK2, sorted on GFP, and cultured for 2 additional days. (B) The percentages of Tpo-dependent or -independent growth of megakaryocyte colonies derived from CFU-Mk (colony-forming unit megakaryocyte) in the presence of shRNA directed against MPL/TpoR or JAK2 compared with scramble sequence. Results are means ± SEM. Student t test: ***P < .0001.