 This is an open label, randomized study comparing the efficacy and safety of randomized 2:1 INC424INCB018424 tablets versus best-available therapy, as selected by the investigator. The purpose is to compare the efficacy, safety and tolerability of INC424INCB018424 given twice daily to the best-available therapy, in subjects with primary myelofibrosis PMF, post polycythemia vera myelofibrosis PPV-MF or post essential thrombocythemia myelofibrosis PET-MF. 