A drug used to treat polycythemia vera in patients who cannot be treated with or have not gotten better with hydroxyurea. It is also used to treat certain types of myelofibrosis. It is being studied in the treatment of other blood diseases and some types of cancer. Jakafi blocks a protein called JAK, which may help keep abnormal blood cells or cancer cells from growing. It is a type of tyrosine kinase inhibitor. Also called ruxolitinib phosphate.