IDH1 and central nervous system cancer: If the immunostain is negative, sequencing of IDH1 and IDH2 can be performed to assess for less common IDH1 mutations, or mutations in IDH2. These non-canonical alterations are very rare in patients over 54, so sequencing is generally not necessary in older patients with glioblastoma histology and no history of a prior lower grade glioma [25, 47, 48].