7971
These observations raise the intriguing possibility that , while loss of neurofibromin function is necessary for NF1 - associated low - grade astrocytoma development , additional genetic changes may be required for full penetrance of the human brain tumor phenotype .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
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