Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.
Q: Tumors of the central nervous system are the most frequent solid tumors in childhood. With 30-40% of this heterogenous group, low-grade astrocytomas represent the most common subtype. Neurofibromatosis type 1 (NF1) is strongly associated with the development of pilocytic astrocytoma (PA), frequently appearing as optic glioma. Neurofibromatosis 1 gene (NF1 ) fulfills the criteria of a tumor suppressor gene and is deleted or mutated heterozygously in patients with NF1. This suggests an involvement in the development of PA. To clarify whether silencing of NF1 by promoter methylation plays a role in PA and especially in optic glioma, the authors investigated the methylation status in 30 PA, 6 of which had optic glioma. However, no methylation was found at the NF1 promoter region in PA. To rule out that silencing of NF1 by promoter methylation is restricted to higher-grade astrocytomas, 15 pediatric WHO II degree and IV degree astrocytomas were analyzed: 12 astrocytomas II and 3 glioblastomas displayed no NF1 promoter methylation. The authors conclude that NF1 silencing by methylation plays no role in low-grade astrocytoma.
A:
Which is the gene mutated in type 1 neurofibromatosis?