G34R/V mutations in H3F3A were identified in a subset of pediatric GBM defined by a unique set of clinical parameters, namely an adolescent/young adult age group, cortical brain location and in consistent association with concurrent mutations in TP53, similar to IDH mutant astrocytomas, and in ATRX (α-thalassemia/mental retardation syndrome-X-linked) [63], which encodes a subunit of a chromatin remodeling complex required for H3.3 incorporation at pericentric heterochromatin and telomeres [16, 42], in all cases initially identified [63]. Here, ATRX is linked to X-linked syndromic intellectual disability.