Considering that the VAF of NF1 was nearly 50%, that the mutation was heterozygous, and that no CNVs were detected in the NF1 gene region, we conclude that this NF1 mutation must have been present in nearly all the tumor cells of primary GBM and SGS, indicating that the two components of SGS may originate from the same ancestral cancer cell. The gene discussed is NF1; the disease is cancer.