IDH1 and astrocytoma (excluding glioblastoma): In the IDH-mutant astrocytoma category, in particular, there is evidence that these processes significantly impact biologic behavior [17, 175, 179, 209], suggesting that forms of genetic and epigenetic instability may be key to understanding the natural history of IDH-mutant astrocytoma, and may have significant prognostic implications as well as the potential to serve as therapeutic targets.