Therapeutically active compounds and their methods of use

A method of treating an advanced hematologic malignancy, wherein the hematologic malignancy is myeloid sarcoma characterized by the presence of a mutant allele of IDH2, comprising administering to the subject in need thereof a tablet comprising a pharmaceutically acceptable carrier and a crystalline form of 2-methyl-1-[(4-[6-(trifluoromethyl)pyridin-2-yl]-6-{[2-(trifluoromethyl)pyridin-4-yl]amino}-1,3,5-triazin-2-yl)amino]propan-2-ol methanesulfonate, wherein the crystalline form is characterized by an X-ray powder diffraction pattern having peaks at 2θ angles of 7.5, 9.3, 14.5, 18.8, 21.3, and 24.8°±0.2°.