Mpl is required in CALR mutant-induced spontaneous growth of megakaryocytic progenitor and thrombocytosis in mice. (A) Lin⁻ cells were purified from C57BL/6 (n = 3) and c-mpl^−/− mice (n = 3) and transduced with retrovirus expressing CALRwt or CALRdel52. GFP⁺ Lin⁻ cells were sorted 2 days later and plated in a fibrin clot assay in presence of SCF, IL-6, and increasing doses of TPO. Curves are means of CFU-MK frequency ± SEM. Bonferroni multiple comparison test: *P < .05, **P < .001, ***P < .0001. (B) Lin⁻ cells purified from C57BL/6 and c-mpl^−/− mice were transduced to express CALRwt and CALRdel52 and engrafted in wild-type (n = 4-5), c-mpl^−/− (n = 5-6), and tpo^−/− (n = 3-4) mice. Platelet counts were determined 7 weeks after engraftment. Results show individuals and the means ± SEM. Student t test: *P < .05. ns, nonsignificant.