CDKN2A and glioblastoma: Specifically, the MTAP gene encoding 5-methylthioadenosine phosphorylase (MTAP), a key enzyme in the methionine salvage pathway that converts a by-product of polyamine synthesis, 5-methylthioadenosine (MTA), back to methionine, is frequently co-deleted with the tumor suppressor gene CDKN2A/B located on chromosome 9p21 [79], for example in Glioblastoma multiforme (GBM).