However, regardless of the test(s) used to estimate HRD, for example, MGMT promoter hyper-methylation and/or IDH status (gliomas, myeloid neoplasms) [20,21,22], specific rearrangement (gene fusions), or accessing the HRD score [10,13,14,19], these tests lack negative predictive value and inadequately address the complex nature of the HRD phenotype. The gene discussed is IDH1; the disease is central nervous system cancer.