CDKN2A and central nervous system cancer: While Ink4a/Arf loci were most frequently lost at the genetic level as determined by PCR, of 40 hPDGFb-driven gliomas arising in Ntv-a Ink4a/Arf-/-Ptenfl/fl or Ntv-a Ink4a/Arf+/-Pten+/fl injected with RCAS-Cre, 28 did not express Pten; of these, 21 retained Pten allele by real-time PCR, while 7 lost it (Figure S4c).