Disclosed herein are methods of treating cancer in a human, where the methods include determining at least one of the following in one or more samples from the human: the presence or absence of an alanine to valine mutation at residue 687 (A687V) in EZH2 in a sample from the human； or the presence or absence of an increased level of H3K27me2 in a sample from the human as compared to a control； and administering to the human an effective amount of the EZH2 inhibitor GSK126 or a pharmaceutically acceptable salt thereof if the A687V mutation is present, or an increased level of H3K27me2 is not present, or both, in the one or more samples, which is indicative of an increased likelihood of increased response rate and/or prolonged progression free survival.
