Myelofibrosis is characterized by hyperactivation of thrombopoietin signaling which induces a RPS14 deficiency that de-regulates GATA1 in megakaryocytes by hampering its mRNA translation. Since mice carrying the hypomorphic Gata1low mutation, which reduces the levels of Gata1 mRNA in megakaryocytes, develop myelofibrosis1 (Zingariello M. et al. 2015), we investigated whether the thrombopoietin axis is hyperactive in this model. Gata1low mice contained 2-times more Tpo mRNA in liver and TPO in plasma than wild-type littermates. Furthermore, Gata1low LSKs expressed levels of Mpl mRNA (5-times greater than normal) and protein (2-times lower than normal) similar to those expressed by LSKs from TPO-treated wild-type mice. Gata1low marrow and spleen contained more JAK2/STAT5 than wild-type tissues, an indication that these organs were reach of TPO-responsive cells. Moreover, treatment of Gata1low mice with the JAK inhibitor ruxolitinib reduced their splenomegaly. Also in Gata1low mice activation of the thrombopoietin/MPL axis was associated with a RSP14 deficiency and a discordant microarray ribosome signature (reduced RPS24, RPS26 and SBDS expression). Finally electron microscopy revealed that Gata1low megakaryocytes contained poorly developed endoplasmic reticulum with rare polysomes. In summary, Gata1low mice are a bona-fide model of myelofibrosis which recapitulates the hyperactivation of the TPO/MPL/JAK2 axis observed in megakaryocytes from myelofibrotic patients.