For example, this approach can be used when knowing the presence or absence of a single genetic change will provide a diagnosis (see the discussion above of PCNSL and brainstem tumors), direct targeted therapy, i.e., MEK inhibitors in BRAF V600E mutated CNS melanoma (Melms et al., 2018), or provide prognostic information, i.e., TERT promoter variants in malignant gliomas (Shankar et al., 2015). The gene discussed is TERT; the disease is malignant glioma.