A method of increasing radiation sensitivity of tumor cells in a subject is disclosed. The method includes the step of administering to the subject an effective amount of a glutaminase inhibitor, such as CB-839 or BPTES. A method of treating a tumor in a subject is also disclosed, which includes the steps of administering to the subject an effective amount of a glutaminase inhibitor and exposing the tumor to radiation in a dose effective to reduce a size of the tumor.
