RB1 and glioblastoma: In the case for genes with putative tumor suppressive role, frequencies of inactivating mutations in GBM are generally low except a few well-known bona fide tumor suppressor genes such as p53, NF1, RB, and PTEN. For example, CDKN2A is the well-known tumor suppressor and homozygous deletion of this gene is frequently detected in GBM, but there is little report for inactivating mutation.