CDKN2A and pilomyxoid astrocytoma: CDKN2A deletion: Of 19 patients with pilocytic or pilomyxoid astrocytomas who had CDKN2A deletion testing performed on all paired surgical specimens, 13 (68%) were found to have a hemizygous deletion in at least one tumor sample, including several with low-level deletions (all above the testing laboratory- established threshold for positivity by FISH, requiring CDKN2A deletions in > 12% of 100 cells or > 11% of 200 cells).