H3 K27M-mutant, WHO grade 4 diffuse midline gliomas are defined as a diffuse glioma located in midline structures, such as the thalamus, pons, brainstem and spinal cord, and carrying a lysine-to-methionine mutation at amino acid 27 of histone H3.3 (encoded by H3F3A) or histone H3.1 (encoded by HIST1H3B and HIST1H3C)1. This evidence concerns the gene H3C3 and diffuse midline glioma.