The invention relates to inhibitors of EZH2 for use in the treatment of cancers characterised by expression of mutated histone H3 having a mutation of amino acid number 27. The invention also relates to methods for predicting the efficacy of treatment of a cancer with an inhibitor of EZH2 by determining whether the cancer cells contain a gene encoding p16INK4A, wherein the presence of a gene encoding p16 INK4A is indicative of efficacy of treatment of the cancer with an inhibitor of EZH2.