JAK2 and acquired polycythemia vera: Significantly, a small molecule antagonist of MPL preferentially inhibited JAK2 V617F-positive proliferation of PV HSC both in vitro and in vivo compared to normal HSC (73), while in vitro, PV HPC and murine cell lines expressing MPL (55)] or CALR (4, 37) mutations were still THPO-responsive despite the presence of constitutively-activated JAK2.