NF1 and brain neoplasm: These mutations were chosen because they create different brain tumor phenotypes in genetically engineered Nf1 mutant mice: despite the fact that all three mutations lead to premature termination (two stop codons and one frameshift mutation) and increased RAS activity, the Cys383X mutation results in lower optic glioma penetrance (Guo et al., 2019) than that for the Thr1145Val_FS mutation (J.C., unpublished), whereas the Arg681X mutation exhibits greater tumor proliferation (Toonen et al., 2016) than that for the Thr1145Val_FS mutation (J.C., unpublished).