Abstract: This invention relates to methods of treating cancer in a subject such as a human and determining at least one of the following in a sample from the subject, such as a human: (a) the presence or absence of a mutation at the alanine 677 (A677) residue in EZH2; or (b) the presence or absence of a mutation at the tyrosine 641 (Y641) residue in EZH2; or (c) the presence or absence of an increased level of H3K27me3 as compared to a control, and administering to said human an effective amount of an EZH2 inhibitor or a pharmaceutically acceptable salt thereof if at least one of the A677 mutation, Y641 mutation, or increased level of H3K27me3 is present in the sample.