The invention relates to determining the presence of an EZH2 gene mutation in a sample from a subject and inhibition of wild-type and certain mutant forms of human histone methyltransferase EZH2, the catalytic subunit of the PRC2 complex which catalyzes the mono-through tri-methylation of lysine 27 on histone H3 (H3-K27). In one embodiment the inhibition is selective for the mutant form of the EZH2, such that trimethylation of H3-K27, which is associated with certain cancers, is inhibited. The methods can be used to treat cancers including follicular lymphoma and diffuse large B-cell lymphoma (DLBCL). Also provided are methods for identifying small molecule selective inhibitors of the mutant forms of EZH2 and also methods for determining responsiveness to an EZH2 inhibitor in a subject.