In the remaining two patients–BC22 (astrocytoma, IDH-mutant, CNS WHO grade 4 with a germline heterozygous MSH2 variant) and BC86 (embryonal rhabdomyosarcoma and prior glioblastoma with a germline POLE variant)–with no apparent genetic second hit, we found high TMB and COSMIC mutational signatures consistent with defective MMR and defective polymerase epsilon exonuclease proofreading, respectively (Fig. 2a and e). The gene discussed is IDH1; the disease is embryonal rhabdomyosarcoma.