The study investigates the impact of a gain-of-function mutation, Lys27Met (K27M), in human histone H3 on epigenetic regulation and tumor development. K27M mutations are commonly found in pediatric diffuse intrinsic pontine gliomas (DIPGs) and supratentorial glioblastoma multiforme (GBMs). The research shows that DIPGs with the K27M mutation exhibit significantly lower levels of trimethylated lysine 27 (H3K27me3) on histone H3. Transgenic expression of H3K27M in cell lines and mouse models reduces H3K27me3 levels and increases acetylated H3K27 (H3K27ac). Mechanistically, H3K27M inhibits the enzymatic activity of the Polycomb repressive complex 2 (PRC2) by interacting with the EZH2 subunit. Additionally, other lysine-to-methionine substitutions at H3K9 and H3K36 also reduce methylation levels by inhibiting SET-domain enzymes. The study proposes that K-to-M substitutions may alter epigenetic states in various pathologies, suggesting a potential mechanism for cancer development.The study investigates the impact of a gain-of-function mutation, Lys27Met (K27M), in human histone H3 on epigenetic regulation and tumor development. K27M mutations are commonly found in pediatric diffuse intrinsic pontine gliomas (DIPGs) and supratentorial glioblastoma multiforme (GBMs). The research shows that DIPGs with the K27M mutation exhibit significantly lower levels of trimethylated lysine 27 (H3K27me3) on histone H3. Transgenic expression of H3K27M in cell lines and mouse models reduces H3K27me3 levels and increases acetylated H3K27 (H3K27ac). Mechanistically, H3K27M inhibits the enzymatic activity of the Polycomb repressive complex 2 (PRC2) by interacting with the EZH2 subunit. Additionally, other lysine-to-methionine substitutions at H3K9 and H3K36 also reduce methylation levels by inhibiting SET-domain enzymes. The study proposes that K-to-M substitutions may alter epigenetic states in various pathologies, suggesting a potential mechanism for cancer development.