ATRX and astrocytoma (excluding glioblastoma): IDH mutations define distinct tumors and the diagnosis of astrocytomas and oligodendrogliomas is no longer based on pure histological appearance, but on the presence of an IDH mutation (both entities) and on the presence (oligodendroglioma) or absence of 1p/19q codeletion which is often associated with loss of ATRX (astrocytoma).