Sequencing of pediatric gliomas has identified missense mutations Lys27Met 
(K27M) and Gly34Arg/Val (G34R/V) in genes encoding histone H3.3 (H3F3A) and H3.1 
(HIST3H1B). We report that human diffuse intrinsic pontine gliomas (DIPGs) 
containing the K27M mutation display significantly lower overall amounts of H3 
with trimethylated lysine 27 (H3K27me3) and that histone H3K27M transgenes are 
sufficient to reduce the amounts of H3K27me3 in vitro and in vivo. We find that 
H3K27M inhibits the enzymatic activity of the Polycomb repressive complex 2 
through interaction with the EZH2 subunit. In addition, transgenes containing 
lysine-to-methionine substitutions at other known methylated lysines (H3K9 and 
H3K36) are sufficient to cause specific reduction in methylation through 
inhibition of SET-domain enzymes. We propose that K-to-M substitutions may 
represent a mechanism to alter epigenetic states in a variety of pathologies.