Pediatric glioblastomas (GBM) are highly aggressive and lethal tumors. Recent 
sequencing studies have shown that ~30 % of pediatric GBM and ~80 % of diffuse 
intrinsic pontine gliomas show K27M mutations in the H3F3A gene, a variant 
encoding histone H3.3. H3F3A K27M mutations lead to global reduction in 
H3K27me3. Our goal was to develop biomarkers for the histopathologic detection 
of these tumors. Therefore, we evaluated the utility of measuring H3K27me3 
global reduction as a histopathologic and prognostic biomarker and tested an 
antibody directed specifically against the H3.3 K27M mutation in 290 samples. 
The study cohort included 203 pediatric (including 38 pediatric high-grade 
astrocytomas) and 38 adult brain tumors of various subtypes and grades and 49 
non-neoplastic reactive brain tissues. Detection of H3.3 K27M by 
immunohistochemistry showed 100 % sensitivity and specificity and was superior 
to global reduction in H3K27me3 as a biomarker in diagnosing H3F3A K27M 
mutations. Moreover, cases that stained positive for H3.3 K27M showed a 
significantly poor prognosis compared to corresponding negative tumors. These 
results suggest that immunohistochemical detection of H3.3 K27M is a sensitive 
and specific surrogate for the H3F3A K27M mutation and defines a prognostically 
poor subset of pediatric GBM.