Additional Study Information: This study will test an experimental drug, momelotinib, for the treatment of primary myelofibrosis, post-polycythemia vera myelofibrosis, or post-essential thrombocythemia myelofibrosis. Primary myelofibrosis, post-polycythemia vera myelofibrosis, or post-essential thrombocythemia myelofibrosis are all myeloproliferative disorders; which are serious bone marrow disorders that disrupt your body's normal production of blood cells. The purpose of this study is to see if momelotinib is effective in reducing your need for red blood cell transfusions due to your myelofibrosis, post-polycythemia vera myelofibrosis or post-essential thrombocythemia myelofibrosis, and to conduct testing on your blood and tissue samples to increase our understanding of how momelotinib works in your body.