IDH1 and astrocytoma (excluding glioblastoma): Furthermore, diffuse astrocytomas with IDH mutations are most likely to harbor mutations in ATRX and P53 [26], and cIMPACT-NOW Update 2 states that a definite loss of ATRX nuclear expression and/or p53 immunopositivity is sufficient for the diagnosis of astrocytoma without the need for a 1p/19q test [27].