INTRODUCTION. GBM is the most common and lethal primary malignant brain tumor. Homozygous deletion of the MTAP gene occurs in 50% of all GBM cases, rendering it one of the most frequent genetic alterations in GBM (1,2). MTAP is often co-deleted with the neighboring tumor suppressor gene, cyclin-dependent kinase inhibitor 2A (CDKN2A), lending to the perception of MTAP deletion as a passenger event..