The histone methyltransferase PRC2 plays a central role in genomic stability and cellular development. Consequently, its misregulation has been implicated in several cancers. Recent work has shown that a histone H3 mutant, where the PRC2 substrate residue Lys27 is replaced by methionine, is also associated with cancer phenotypes and functions as an inhibitor of PRC2. Here we investigate the mechanism of this PRC2 inhibition through kinetic studies and photo-cross-linking. Efficient inhibition is dependent on (1) hydrophobic lysine isosteres blocking the active site, (2) proximal residues, and (3) the H3 tail forming extensive contacts with the EZH2 subunit of PRC2. We further show that naturally occurring post-translational modifications of the same H3 tail, both proximal and distal to K27M, can greatly diminish the inhibition of PRC2. These results suggest that this potent gain of function mutation may be "detoxified" by modulating alternate chromatin modification pathways.