In the revised World Health Organization 2016 classification of central nervous system tumors, "diffuse midline glioma, H3 K27M-mutant" has been added as a new diagnostic entity. However, some confusion exists concerning this diagnostic entity because H3 K27M-mutant diffuse midline glioma is diagnosed with grade IV regardless of morphologic phenotype. Furthermore, the significance of H3 K27M mutation in tumors that aren't typical "diffuse midline glioma, H3 K27M-mutant," such as those with an unusual location and nontypical histology, remains unclear.