Activating mutations in JAK1 and JAK2 have been described in patients with various hematologic malignancies including acute lymphoblastic leukemia and myeloproliferative neoplasms, leading to clinical trials with JAK inhibitors. While there has been a tremendous effort towards the development of specific JAK inhibitors, mutations conferring resistance to such drugs have not yet been observed.Taking advantage of a model of spontaneous cellular transformation, we sequenced JAK1 in selected tumorigenic BaF3 clones and identified 25 de novo JAK1 activating mutations, including 5 mutations already described in human leukemias. We further used this library of JAK1 mutation-positive cell lines to assess their sensitivity to ATP-competitive inhibitors.While most JAK1 mutants were sensitive to ATP-competitive JAK inhibitors, mutations targeting Phe958 and Pro960 in the hinge region of the kinase domain rendered JAK1 constitutively active but also resistant to all tested JAK inhibitors. Furthermore, mutation of the homologous Tyr931 in JAK2 wild-type or JAK2 V617F mutant found in patients with myeloproliferative neoplasms also conferred resistance to JAK inhibitors, such as INCB018424, which is currently in clinical use.Our data indicate that some activating mutations not only promote autonomous cell proliferation but also confer resistance to ATP-competitive inhibitors. In vivo, such a mutation can potentially occur as primary JAK-activating mutations but also as secondary mutations combining oncogenicity with drug resistance.