IDH-mutant astrocytomas often harborATRXand/orTP53mutations (22), and immunohistochemical stains demonstrating loss of nuclear ATRX expression and/or aberrant p53 staining (staining in the majority of tumor nuclei, >50%, or less likely complete absence) can be used as surrogate markers. Here, IDH1 is linked to astrocytoma (excluding glioblastoma).