The present invention relates to crystalline 2,5-dione-3-(1-methyl-1H-indol-3-yl)-4-[1-(pyridin-2-yl-methyl)piperidin-4-yl]-1H-indol-3-yl]-1H-pyrrole mono-hydrochloride salt, a pharmaceutical formulation containing said salt and to methods for treating cancer and for inhibiting tumor growth using said salt.