Provided are crystalline forms of 2-Methyl-l-[ (4-[ 6-(trifluoromethyl) pyridin-2-yl]-6-{[2-(trifluoromethyl)pyridin-4-yl]amino}-1,3,5-triazin-2- yl)amino]propan-2-ol mesylate useful for treating cancer and methods of treating cancer, comprising administering to a subject in need thereof said compound.