Provided herein is N-((4,6-dimethyl-2-oxo-1,2-dihydropyridin-3-yl)methyl)-5-(ethyl (tetrahydro-2H-pyran-4-yl)amino)-4-methyl-4'-(morpholinomethyl)-[1,1'-biphenyl]-3-carboxamide hydrobromide for use in the treatment of a cell proliferative disorder of the hematologic system. Also provided herein is a particular polymorph form of this compound and its use.