Ours is the first study to report that a) OX40L and 41BBL expression increases in CRC cells when DNMTs are inhibited, b) expression of OX40L and 41BBL increases in human CRC cells when HDACs are inhibited, c) HDAC inhibition in CRC cells can increase the activation and survival of T cells, and d) radiation treatment of tumor cells results in epigenetic modification of the histones in the promoter of the costimulatory gene 41BBL. The gene discussed is HDAC9; the disease is colorectal carcinoma.